Wait until old ComposerClient is fully destroyed before recreating

Possible race condition where EventCallbacks are registered before
old implementation deregisters on destruction.  Stems from fact that
strong pointer destruction does not mean the object has completely run
its destructor method

Bug: 135210708
Test: build, boot, VtsHalGraphicsComposerV2_1TargetTest
Change-Id: I0851f1197d8341854f5bdac5fbb08553fc32b710