Synchronize on the SparseArray object in PendingTempAllowlists.

The field mPendingTempAllowlist in PendingTempAllowlists is a
SparseArray.

In ActivityManagerService, the lock protection for PendingTempAllowlists
object is:
 @CompositeRWLock({"this", "mProcLock"})
    final PendingTempAllowlists mPendingTempAllowlist

Which means the mPendingTempAllowlist object can be read when either
"this" lock or "mProcLock" lock is held. But the read-operation of
SparseArray such as indexOfKey() and size() etc, actually mutate the
SparseArray by calling SparseArray.gc(). This makes @CompositeRWLock not
to be compatible with SparseArray.

Since we can not make SparseArray thread-safe, also we want to maintain
the semantic of @CompositeRWLock, we can make
PendingTempAllowlists thread-safe at least.

Bug: 193788840
Test: Regression test.
Change-Id: Ie1c239ad27d1fd6b76676951b470605513848b20