Fix deadlock in AccessCheckDelegateImpl.

We shouldn't call into packages when holding our own lock. We can simply
temporarily release our lock when we need to call into packages to get
the UID.

We only call into packages when needed because there's a non-trivial
performance cost if we do that.

It's also okay for us to temporarily release our lock in this case
because even if mOverridePermissionStates changes while we aren't
holding the lock, we'll still return a valid permission check result, as
we don't have atomicity guarantee between shell identity and override
permission states which are two APIs, and we can accept the small
one-time performance penalty of needlessly fetching the UID in the
unlikely case there was an override permission state that's now gone.

Fixes: 429911389
Flag: EXEMPT bug fix
Test: presubmit
Change-Id: I5b620285741f101ea933b60be88d4b8808566a74