Make onGrantDefaultRoles() run on worker thread.

This re-initialization can happen relatively frequently when packages
or package component state changes, so if we don't do synchronization
we may run into concurrency issues when modifying state in
RoleManagerService.

Bug: 110557011
Test: build
Change-Id: I26dfe1cba9d66d7ee5b9d61480e93b792f2b1f8a