Purge proto tombstones when apps are uninstalled.

In preparation for making tombstones available to apps, make sure we
clean up after ourselves, so that when we recycle UIDs, we don't
accidentally give out old tombstones from unrelated previous apps.

Bug: http://b/159164105
Test: manual
Change-Id: I76646b6e98bc9019bb8d978fa416303a617a8238