Support Notification bitmap offloading.

Introduce a BitmapOffloadService in system_server that can take a Bitmap
and serve it as a Uri through a ContentProvider. This allows the Bitmap
to be written to disk, and to be loaded back into memory when needed.

The ContentProvider will only allow system_server to insert, modify and
delete entries. Users of the offloader must provide a PermissionChecker
interface to determine which callers are allowed to retrieve the
Bitmap associated with each entry.

The offloader does not persist the Bitmaps; they are wiped on every
system_server start. It will also only allow offloading Bitmaps if the
device has enough free storage space.

Modify NotificationManagerService to offload BigPicture Bitmaps, if
the corresponding flag is enabled.

Bug: 398153219
Flag: android.service.notification.notification_bitmap_offloading
Test: atest CtsNotificationTestCases
Change-Id: I531d3e81c8733f913ec7638f3877d11bb1229e88