Improve bitmap IPC

Make all received bitmaps immutable as this
improves re-use for subsequent transfers.

Remove minimum size to stay on ashmem, as
Parcel's writeBlob already has similar logic.
This effectively reduces the minimum size to 16kB.

Bug: 148301859
Test: device booted, a notification showed up
Change-Id: I2fafb0989944c0ef82de6edcb4924056a592cf66