Race condition in Toast.

1. mTN.mNextView is set in toast.show() but can be invalidated by posted
runnable mTN.mHide. If application call toast.cancel() right before
toast.show(), this race condition can occur.

2. Removes superfluous TN.hide() if toast.cancel() is called.

Change-Id: I554c5d44c59e18f5066f6175faec3e256b4e40d6
update SystemUI's proguard.flags

Change-Id: I5aa73eddd3fea6cddeddaf94e202d8de870e7df5