Instead of storing AppIconDrawable store bitmap.

Storing the AppIconDrawable itelf currently introduces around 2mb of
extra memory usage per window. Replace AppIconDrawable usage with bitmap
usage so that we don't need as much memory. Bitmap uses 45x45x8 bytes of
memory only.

Bug: 336026116
Test: Manual | Compare memory usage through heap dumps and perfetto
Change-Id: Ibed72b837aff08389506dfd65b1d4e486c7f3f9a