android: read the activity's own arguments, since Qt does not deliver them

`am start -n <pkg>/org.qtproject.qt.android.bindings.QtActivity --esa
applicationArguments "--workspace,/sdcard/x"` left the window on its default
workspace on the NX789J (the plain-string extra too), so the extra is read
from the intent directly. That is what makes `cst-agent-app --uitest
--workspace DIR` reachable on a device with no keyboard -- and adb cannot
inject touch into this window either (tap, touchscreen tap, mouse tap and
motionevent all do nothing; key events do work), so it is the only way in.

Everything degrades to "no arguments": an invalid object, a missing method or
a context that is not an activity all come back empty.

app/src/core/ToolProvider.h

@@ -97,6 +97,12 @@ bool hasAllFilesAccess();
  * dialog for it). A desktop, a phone that already has it or one too old to
  * know about it returns "" and nothing opens. */
 QString requestAllFilesAccess();
+/* The arguments the Android activity was started with (the
+ * "applicationArguments" intent extra): the way
+ * `cst-agent-app --uitest --workspace DIR` is reachable on a device, since
+ * Qt's own plumbing for that extra does not reach this build. Empty
+ * everywhere else, and empty on a device that was not started with any. */
+QStringList activityArguments();
 
 }  // namespace app