Throttle requests to ActivityManager.getProcessMemoryInfo().

This is very expensive and needs to run in the system process, we
don't want apps abusing it.

Also don't allow apps to get information about anything but their
own process, unless they have the appropriate privileged permissions.

Bug: 112537519
Test: manual
Change-Id: I01997d6f888341e8eb2afe6a69545dd5be013744