Add an ID field to `class Bitmap`

Bug: 369619160
Flag: NONE no public API change

To be able to better reason about bitmaps, we added a private mId
field to `class Bitmap`.  This allows the same ID to be used in
heapdump and in log/debugging code for better association.

Change-Id: I5f775167266c87428c717014fa01cb73c59a44c9