Clear image caches in reaction to system VM warnings

* java/org/gnu/emacs/EmacsNative.java (onLowMemory):

* java/org/gnu/emacs/EmacsService.java (onLowMemory): New
function.

* src/android.c (android_on_low_memory, onLowMemory): New
functions called when a VM caution is registered.  Clear
the image cache and run garbage collection.

java/org/gnu/emacs/EmacsNative.java

@@ -96,6 +96,9 @@ public final class EmacsNative
      thread, then return.  */
   public static native void shutDownEmacs ();
 
+  /* Garbage collect and clear each frame's image cache.  */
+  public static native void onLowMemory ();
+
   /* Abort and generate a native core dump.  */
   public static native void emacsAbort ();