Assert the results of flushIfNeeded in freeze notifications

Propagate the return status_t from flushCommands to flushIfNeeded and
assert the results in addFrozenStateChangeCallback and
removeFrozenStateChangeCallback.

This will help us validate that flushIfNeeded really fixed the previous
assertion failures (rather than simply hiding them)

Bug: 438625168
Test: binderLibTest
Flag: EXEMPT bugfix
Change-Id: Ifdccb8e6599abfd979b39f53d4bbd1ff79108fc0