HALO: Option to pause the underlying app (1/2)

- adds an option to pause the active app when using HALO
- this is enabled by default for devices with low memory
- (currently: <640MB RAM available to the kernel)

Change-Id: If450d81aa3f6b660a518c1b11a451a5b113f3817

Conflicts:
	services/java/com/android/server/am/ActivityStack.java