android: Drop JavaPanZoomController.mMainThread

Instead call the logic to get the main thread directly
in JavaPanZoomController.checkMainThread.

Change-Id: I5fe02ba038b0e4ea954012e2a7296f59113e1eb6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/193960
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

android/source/src/java/org/mozilla/gecko/gfx/JavaPanZoomController.java

@@ -74,7 +74,6 @@ class JavaPanZoomController
     private final Axis mX;
     private final Axis mY;
     private final TouchEventHandler mTouchEventHandler;
-    private final Thread mMainThread;
     private final LibreOfficeMainActivity mContext;
 
     /* The timer that handles flings or bounces. */
@@ -99,7 +98,6 @@ class JavaPanZoomController
         mY = new AxisY();
         mTouchEventHandler = new TouchEventHandler(view.getContext(), this);
 
-        mMainThread = mContext.getMainLooper().getThread();
         checkMainThread();
 
         setState(PanZoomState.NOTHING);
@@ -124,7 +122,7 @@ class JavaPanZoomController
 
     // for debugging bug 713011; it can be taken out once that is resolved.
     private void checkMainThread() {
-        if (mMainThread != Thread.currentThread()) {
+        if (mContext.getMainLooper().getThread() != Thread.currentThread()) {
             // log with full stack trace
             Log.e(LOGTAG, "Uh-oh, we're running on the wrong thread!", new Exception());
         }