Fix binderLibTest to flush commands

binderLibTest calls addFrozenStateChangeCallback and expects the
underlying binder command to be flushed and a frozen notification to be
delivered immediately; however this binder behavior is changing with the
rollout of RELEASE_LIBBINDER_FREEZE_USE_FLUSH_IF_NEEDED, with which
IPCThreadState would no longer flush immediately on certain cases, e.g.
when addFrozenStateChangeCallback is called from the main thread, which
is what's happening in binderLibTest.

This change fixes the issue temporarily by making the test call
flushCommands directly. Separate work will follow to add proper binder
support for addFrozenStateChangeCallback being called from the main
thread.

Bug: 435986995
Test: atest BinderLibTest
Flag: TEST_ONLY
Change-Id: I65efcb7ba9c54dc251c6c338c1bbea36de416935