Require that persistable Uri permissions be taken.

Change our Intent flag to indicate that a Uri permission grant is
persistable, but don't actually persist it until explicitly taken by
the receiving app.  This prevents apps from spamming each other if
persisted permissions aren't really required.

Remember the last time a persisted grant was taken by an app, and
use this to prune away the oldest grants when the number of grants
grows too large.  Allow apps to query persisted grants they are
holding, and allow them to release previously persisted grants. Add
public UriPermission class to return grant details and timestamp.

Track various permission strengths separately, and combine together
after each mutation pass.  Persistable grants are currently treated
like global grants, but they could be moved to have owners in the
future.  Require that grant holders trying to extend a persistable
permission actually hold a persistable permission themselves.

Bug: 10835779
Change-Id: I95b2f797c04ce7fd2612f9a644685dbd44e03759