Add compressed bitmaps to be included in `am dumpheap`

Bug: 328443220

`android.graphics.Bitmap` class used to have a field of byte array
`mBuffer` for its content. This allowed the bitmap content to be
included as part of `am dumpheap`. However, this field was removed
when Bitmap was migrated to use native memory.

This CL allows contents of bitmaps to be compressed and included as
part of `am dumpheap`, with added command line switch '-b <format>'.
For example, the command below will include the contents of the
bitmaps compressed in PNG format as part of the heap dump.

  `am dumpheap -b png com.google.android.apps.photos`

This is done with a few key changes below:

  1. Every bitmap instance created will be tracked by a static
     `WeakHashMap`. This is so that 1) the bitmap instances are
     used as weak keys and can be garbage collected normally,
     and 2) when a bitmap instance is garbage collected, its
     entry in `WeakHashMap` will also be removed, so the size
     of the map itself is limited

  2. A static field `Bitmap.dumpData` is introduced, and will
     record every bitmap's `nativePtr` and its compressed
     content when bitmap dump is enabled during a heap dump

  3. `Bitmap.dumpData` will be cleared after the heap is dumped,
     the recorded information as well as buffers with compressed
     contents will be garbage collected thereafter.

Change-Id: I37b6ea6b947565d1ac5a6bbc5b462c3ceedebec1