Cache resolved details about remote callers.

The logic in MediaProvider is technically correct, but it's sometimes
inefficient in calling into the OS multiple times with the same
questions, such as validating getCallingPackage().

To mitigate this overhead, and start paving the way for more dynamic
delegation of permission checks, collect these details into a
LocalCallingIdentity object.  We carefully perform all permissions
checking against this new object, and avoid using any other
thread-local values from ContentProvider or Binder.

Local tests show this CL improves performance of a test app that
takes 100 rapid shots by 37%.

This change is a no-op refactoring.

Bug: 130758409, 115619667
Test: atest --test-mapping packages/providers/MediaProvider
Change-Id: If250a7675f2246cd10881acf615619d6d6061f3d