Deallocate offscreen buffer for non-HW-accel draw

Follow up to I5bc980c4dc41bc304be3c190934c1f7c55c9d244

This frees up 800KB on occam_svelte when LatinIME is in background.
Confirmed the performance effect in keyboard-bring up speed was unperceivable.

bug: 8967766
Change-Id: I0fab179c61179564bd3a0349c9825e7d61f7e1f2

java/src/com/android/inputmethod/keyboard/KeyboardView.java

@@ -605,4 +605,8 @@ public class KeyboardView extends View {
         super.onDetachedFromWindow();
         freeOffscreenBuffer();
     }
+
+    public void deallocateMemory() {
+        freeOffscreenBuffer();
+    }
 }

java/src/com/android/inputmethod/keyboard/MainKeyboardView.java

@@ -1322,7 +1322,9 @@ public final class MainKeyboardView extends KeyboardView implements PointerTrack
         }
     }
 
+    @Override
     public void deallocateMemory() {
+        super.deallocateMemory();
         mGestureTrailsPreview.deallocateMemory();
     }
 }