Fix unregAll throwing Concurrent exception

unregAll does iterate over ContextMap.mApps without synchronization,
and then call unregisterClient, which also iterate over same variable,
and remove element. This cause ConcurrentModificationException. To fix
that, make sure mApps access is always guarded by synchronized block.
Also make mApps into private variable, to make sure no one would make
similar errors in the future.

Bug: 34805290
Test: manual
Change-Id: I6254c809423ecc3901b64f1a2d60324efd1c4834