Don't lose BR_RELEASE/BR_DECREFS commands.

BR_RELEASE/BR_DECREFS commands are stored in mPendingStrongDerefs
and mPendingWeakDerefs, respectively. During processPendingDerefs(),
we actually execute the corresponding decStrong()/decWeak() operations.

The problem is that when we're done, we clear() the mPending vectors
without checking if new entries have been added to them. This can
happen, because decStrong()/decWeak() might cause destructors to run,
which in turn can invoke outgoing transactions that may result in
a BR_RELEASE/BR_DECREFS being queued.

Bug: 63079216
Test: binderLibTest
Change-Id: Ib1deca3f317f8b5068b4b9eddfc4219b9ec87740