Halo: Add master switch to disable Halo (1/2)

Change-Id: Ifb0f4e8abdce1ed3d4cc4c973edc08794781db64

Conflicts:
	core/java/android/provider/Settings.java
Halo PS 16 -> 17

Change-Id: I11b694ecd6a46f745596e32d81479b863ff6c232
PS17: Get the correct activity stack: this makes all windows in the same stack to get the floating mode
fix build error for halo
fix build error for halo