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

Change-Id: If91f950986bd816d5571a0d51a7cb9522226e270