frameworks/base: Fix for the race in Wallpaperservice

Wallpaperservice has a race among message handler of
DO_DETACH and service's onDestroy. In certain cases,
the engine kept track in mActiveEngines is removed by
message handler of DO_DETACH and service's onDestroy
doesn't get a entry in mActivieEngines and isn't able
to invoke detach. This keeps the broadcast receiver,
mReceiver active and is unregistered by the framework
and a corresponding leak is reported.

  Later, the message handler of DO_DETACH continues
and invokes detach on mEngine, which attempts to
unregister mReceiver and framework throws an exception
"Receiver not registered: android.service.wallpaper.
WallpaperService$Engine$1@40874580" and causes the
framework to reboot.

  In case of system_server, WindownManagerPolicy and
android.server.ServerThread contest for access of
shared members, mActiveEngines and mEgine. Fix is to
protect the critical section via synchronized block.

Change-Id: Ice79cb8344fb54bcc6e9f76f4b143daac2608d7b