SurfaceComposer: avoid data races by holding lock more

There's a data race between SurfaceComposerClient::dispose and
SurfaceComposerClient::getClient which I think is the likely culprit
behind the linked bug's weird refcounting error.

Bug: 408094875
Flag: EXEMPT small bug
Test: presubmit
Change-Id: Id705453200344bb4dc1c88f07dd03504f74cf1b1