Keep source bitmap id for received ashmem bitmaps

Bug: 369619160
Flag: NONE - private API for debugging

When transfering bitmap mId from sending side to receiving side,
a same bitmap could be sent multiple times, which ends up with
multiple bitmaps on the receiving end having the same mId.

To keep mId unique while not losing the track of where the bitmap
was originated from, we keep the source mId in a separate source
field, and Bitmap.getSource()/Bitmap.setSource() to retrieve and
specify the source bitmap.

Change-Id: Ic85a0e8324fc0b1bf03b5ed8c1358b8449c00d49