v0.9.3: notification_send posts on the phone, and Java helpers have a home

The tool was the last Android one that never worked on the NX789J: it would
fail with "Notification.Builder.build() failed: no exception detail" and the
shade stayed empty. The detail was the whole story -- Qt clears a Java
exception before C++ gets to look at it, so a build() that throws and a
build() that returns null look exactly the same from here, and the only way
to catch the difference is to let Java catch it.

- app/android/src/xyz/whcself/cstagent/Notifier.java: the builder, the
  PendingIntent, build() and notify() in one try/catch that returns "" or
  the throwable's class and message. androiddeployqt copies app/android/ with
  the rest of the package source and Qt's Gradle template puts its src/ on
  java.srcDirs, so a class dropped there needs no build-script change (the
  first one is this; the foreground service of the next step goes there too).
- AndroidTools.cpp keeps what it can see from C++ (the context, the channel
  on API 26+, the small icon looked up by name, the launch intent) and calls
  Notifier.post(); the result line now carries the Java failure when there is
  one. The javaErrorText() helper of the previous attempt is gone: reading a
  pending exception from C++ is exactly what does not work here.
- On the device: the tool answers "notification sent", the system holds the
  record (channel cst-agent, importance 3, the framework small icon, the tap
  intent back into the app). Earlier attempts in the same session are in the
  transcript next to it: four failures from the old path, then this one.

Docs: docs/android.md gets the Java-helper mechanism (and why the tool has to
be wrapped in Java at all) and no longer lists notification_send as missing;
app/README.md's platform table and the Android note say where a Java class
goes.

Version 0.9.3.

app/android/src/xyz/whcself/cstagent/Notifier.java

@@ -0,0 +1,60 @@
+package xyz.whcself.cstagent;
+
+import android.app.Notification;
+import android.app.NotificationManager;
+import android.app.PendingIntent;
+import android.content.Context;
+import android.content.Intent;
+
+/**
+ * The notification behind the engine's notification_send tool.
+ *
+ * It lives in Java because a Java exception is invisible to C++ by the time
+ * Qt's JNI wrapper is done with the call: Notification.Builder.build() can
+ * throw (a small icon Android refuses, a channel it does not know) and the
+ * JNI call then simply returns null -- the tool could only say "failed, no
+ * exception detail" on a device where the phone's own shade never saw
+ * anything. Catching it here turns the failure into the line the tool shows.
+ *
+ * Called from app/src/platform/android/AndroidTools.cpp through QJniObject.
+ * androiddeployqt copies this file with the rest of the package source
+ * (QT_ANDROID_PACKAGE_SOURCE_DIR, app/android/) and Qt's Gradle template adds
+ * "src" to java.srcDirs, so no build script change is needed for a file
+ * added here.
+ */
+public final class Notifier {
+    private Notifier() {}
+
+    /**
+     * Post one notification. An empty channelId means an API below 26, which
+     * has no channels. Returns "" when it went out, the Java failure
+     * otherwise (class name and message), so the tool result can name it.
+     */
+    public static String post(Context ctx, String channelId, String title,
+                              String text, int iconId, Intent tap, int id) {
+        try {
+            Notification.Builder builder = channelId == null || channelId.isEmpty()
+                    ? new Notification.Builder(ctx)
+                    : new Notification.Builder(ctx, channelId);
+            builder.setContentTitle(title).setContentText(text).setSmallIcon(iconId);
+            if (tap != null) {
+                tap.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK
+                        | Intent.FLAG_ACTIVITY_CLEAR_TOP);
+                PendingIntent pending = PendingIntent.getActivity(
+                        ctx, 0, tap,
+                        PendingIntent.FLAG_IMMUTABLE | PendingIntent.FLAG_UPDATE_CURRENT);
+                builder.setContentIntent(pending);
+            }
+            NotificationManager manager = (NotificationManager)
+                    ctx.getSystemService(Context.NOTIFICATION_SERVICE);
+            if (manager == null)
+                return "no NotificationManager";
+            manager.notify(id, builder.build());
+            return "";
+        } catch (Throwable t) {
+            String message = t.getMessage();
+            return t.getClass().getSimpleName()
+                    + (message == null || message.isEmpty() ? "" : ": " + message);
+        }
+    }
+}