Use WeakReferenes to reduce Bitmap memory footprint

Use HashMap<Integer, WeakReference<Drawable>> instead of just
    HashMap<Integer, Drawable>

Without this, references are still being held for Bitmap drawables,
thereby after few Photo edits, OutOfMemoryException is raised and
Gallery crashes

Conflicts:
	src/com/android/gallery3d/filtershow/filters/ImageFilterBorder.java

Change-Id: If91f950986bd816d5571a0d51a7cb9522226e270