Hide suspended apps from recents

* Introduce a new boolean realActivitySuspended in TaskRecord to
keep track of the suspended state (this also makes sure the
information is persisted across reboots).
* Introduce a new method onPackagesSuspendedChanged
in RecentTasks which flips the realActivitySuspended for each
TaskRecord for the desired user and packages sent
* Register a new broadcast receiver (for SUSPEND/UNSUSPEND)
in ActivityManager#finishBooting which calls
RecentTasks#markPackagesSuspendedStateForUser
* In ActivityManagerService#getRecentTasks filter out
Taskrecords for suspended apps by checking realActivitySuspended

Bug: 22776761
Change-Id: I52e3593ccb2dae223d1ea11d017a6b1626646639