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)

Signed-off-by: Dan Trifan <jimsth@gmail.com>

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

Change-Id: I87e1c091fb21023a010d89a1f27ce50589ac8845