Icon: make Icon's Bitmaps immutable

Because Bitmap mutability is propagated across Parcel and Binder, the
following problem can happen when sending an Icon containing a Bitmap
as part of a Notification:

1. App creates Icon with a mutable Bitmap (1 alloc)
2. App sends Icon to system_server, often creating ashmem Bitmap (2 allocs)
3. system_server converts ashmem Bitmap back to heap Bitmap (3 allocs)
4. system_server sends heap Bitmap to each NotificationListener individually,
converting the heap Bitmap to ashmem each time (3+N allocs)
5. NotificationListener converts ashmem Bitmap to heap Bitmap (3+2N allocs)

This is inefficient. This is especially bad because the API to update
a Notification involves sending a full Notification object, including
Icons, repeatedly, and some apps do that several times per second.

Instead, ensure that all Bitmaps transmitted as part of Icons are
immutable. Instead, we get:

1. App creates Icon, which may copy to immutable Bitamp (1 or 2 allocs)
2. App sends Icon to system_server over ashmem (still 1 or 2 allocs)
3. system_server uses received ashmem Bitmap directly (still 1 or 2 allocs)
4. system_server sends same ashmem Bitmap to NotificationListeners (1 or 2)
5. Each NotificationListener uses the same ashmem Bitmap (1 or 2)

This solves the per-NotificationListener amplification, but it does
not solve sending what is likely the same Bitmap N times per second
when apps update Notifications. That will require either app changes
or Notification API changes to avoid bytewise comparisons for all
Bitmaps.

Test: boot, notifications work, memory in maps/gearhead remains good
Bug: 227920378

Change-Id: If92835f647a76da599f358c7c02888e3e2f59235