Aggresive performance optimizations.

Now that we have LocalCallingIdentity, we can start caching it in
very narrow cases.  We must be careful to not cache too long, since
any changes to granted permissions for the UID mean we need to
re-evaluate any cached answers.

The best middle-ground for this in the Q release is to use an active
camera session as a proxy for when we should create a cache object
and then later invalidate it.  (It's very unlikely that a user
changes permissions while actively using the camera, and this is
a strong signal that the caller is sensitive to performance.)

Many other sprinkled optimizations to avoid extra binder calls into
the OS, such as aggressively caching VolumeInfo related details.

Track IDs that are owned by each LocalCallingIdentity, to speed up
all future security checks.

Dispatch all change notifications asynchronously, and delay them by
several seconds while the camera is being actively used, to give
more important foreground work a fighting chance.  Invalidate
thumbnails asynchronously.

Optimizations to ModernMediaScanner where it's safe to skip the
"reconcile" and "clean" steps when we're focused on a single file
that we successfully scanned.

Local tests show this CL improves performance of a test app that
takes 100 rapid shots by 45%.  (All the collective optimizations
done so far this week add up to a 70% improvement.)

Bug: 130758409
Test: atest --test-mapping packages/providers/MediaProvider
Exempt-From-Owner-Approval: trivial manifest change
Change-Id: I38cc826af47d41219ef44eae6fbd293caa0c01d5