aaudio: protect against null client

AAudioService would fail if a null client was passed.
Two null checks were added. One where we know the null
client first appears. And one where the client is first used
in case other calls are passing null.

Bug: 116230453
Test: Bug has a POC apk that triggers the bug.
Test: Look for messages like:
Test:      AAudio  : BnAAudioService::onTransact() client is NULL!
Change-Id: Id9c4fc154226ab40df97335da8bc9361cfc99a73