Run performInitialGrantsIfNecessaryAsync() on main thread.

performInitialGrantsIfNecessaryAsync() is already made asynchronous,
so it is no longer necessary to post it to a thread pool. After this
change both this method and onRemoveUser() can run on the main thread,
so that we won't run into a race condition with a destroyed user
state.

Bug: 130738316
Test: presubmit
Change-Id: I9f6e4d9badddf5396c5a9aee683b67739c82884a