Kill the task killers.

The ActivityManager.restartPackage() API is now deprecated, and no longer
allows applications to mess up the state of other applications.  This was
being abused by task killers, causing users to think their other applications
had bugs.

A new API is introduced for task killers,
ActivityManager.killBackgroundProcesses(), which allows these applications
to kill processes but only the same amount that the out of memory
killer does, thus causing no permanent damage.  The old restartPackage()
API is now a wrapper for calling this new API.

There is also a new private forceStopPackage() API that is used for the
system's force stop UI which does what the old restartPackage() API did.