Added missing mGlobalLock in system server

The method resetFreezeTaskListReorderingOnTimeout is posted onto a
handler so it loses mGlobalLock. Reacquire the lock when the runnable is
invoked. Without the lock, the code enters very critical code unlocked,
causing race conditions.

Fixes: 135016269
Test: Hard to repro, go/wm-smoke
Change-Id: Iafccc466c1aa22ff6f528e99e68a672e0dc50975