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/EmacsService.java

@@ -321,6 +321,10 @@ public final class EmacsService extends Service
       }
   }
 
+  /* The native functions the subsequent two functions call do nothing
+     in the infrequent case the Emacs thread is awaiting a response
+     for the main thread.  Caveat emptor! */
+
   @Override
   public void
   onDestroy ()
@@ -333,6 +337,14 @@ public final class EmacsService extends Service
     super.onDestroy ();
   }
 
+  @Override
+  public void
+  onLowMemory ()
+  {
+    EmacsNative.onLowMemory ();
+    super.onLowMemory ();
+  }
+
 
 
   /* Functions from here on must only be called from the Emacs