1. Lock prepare() and set() APIs using sequence entry and sequence
exit lock respectively to prevent the critical section being
accessed by other threads.
2. Wait for sequence wait lock on HotPlugEvent thread or any other
binder threads(Eg. dumpsys), before it accesses the critical region.
3. Prevent deadlock during multiple prepare() calls before a set().
Change-Id: I5443675c870ed2967615ec383dd2d59ded30b52b
ef83526f6a