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)

Conflicts:
	core/java/android/provider/Settings.java
	services/java/com/android/server/am/ActivityStack.java

Change-Id: I09c90d615b1b2954293214d5e6107e2e2caab803