Use serial executor for permission operations.

If the methods are called concurrently and we are calling
AppPermissions.persistChanges(), we might be overwriting the state
that have been changed by another thread but not reflected in our
state obtained earlier.

Fixes: 133398449
Test: presubmit
Change-Id: I63f715bafbf0ce24e75246930c7b271ee64b2e1b