PieService: fix possible race condition.

onPieActivation declared as a oneway function (call to it will not
block code execution). If client calls back to gainTouchFocus or
restoreListenerState very quickly then race condtiion happens
and mActive might be false in gainTouchFocus/restoreListenerState.

Set mActive before calling onPieActivation to avoid race condition and
drop the flag if exception happens.

Change-Id: Ied12b7dcbdc1bae2ba0c9fd4e5fa04169e8b2d22