Deallocate gesture trail bitmap when unneeded

This frees up 1.7MB on Nexus 4 (occam), and 900KB on occam_svelte while
LatinIME goes to background.
The keyboard bring-up speed feels acceptable on both devices.

bug: 8967766
Change-Id: I5bc980c4dc41bc304be3c190934c1f7c55c9d244

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

@@ -1370,4 +1370,8 @@ public final class MainKeyboardView extends KeyboardView implements PointerTrack
             drawIcon(canvas, mSpaceIcon, x, y, iconWidth, iconHeight);
         }
     }
+
+    public void deallocateMemory() {
+        mGestureTrailsPreview.deallocateMemory();
+    }
 }

java/src/com/android/inputmethod/keyboard/internal/GestureTrailsPreview.java

@@ -104,7 +104,12 @@ public final class GestureTrailsPreview extends AbstractDrawingPreview {
         freeOffscreenBuffer();
     }
 
+    public void deallocateMemory() {
+        freeOffscreenBuffer();
+    }
+
     private void freeOffscreenBuffer() {
+        mOffscreenCanvas.setBitmap(null);
         if (mOffscreenBuffer != null) {
             mOffscreenBuffer.recycle();
             mOffscreenBuffer = null;

java/src/com/android/inputmethod/latin/LatinIME.java

@@ -894,6 +894,7 @@ public class LatinIME extends InputMethodService implements KeyboardActionListen
         final MainKeyboardView mainKeyboardView = mKeyboardSwitcher.getMainKeyboardView();
         if (mainKeyboardView != null) {
             mainKeyboardView.cancelAllOngoingEvents();
+            mainKeyboardView.deallocateMemory();
         }
         // Remove pending messages related to update suggestions
         mHandler.cancelUpdateSuggestionStrip();