Use realtime priority for systemui and recents(home)

Attempt to reduce jank for the process with important UI.

Note that in trace, the UI/render threads priority will be 98
when they are foreground or animating. Otherwise it will be 100.

Besides, when camera is opened by an uid with top priority, the
current processes with FIFO will be restored to OTHER. And back
to FIFO once the top process closes the camera.

Bug: 288140556
Test: atest ActivityManagerServiceTest#testFifoSwitch
Test: pid=`adb shell pidof com.android.systemui`
      adb shell cat /proc/$pid/stat | awk '{print "priority=" $18}'
      When expanding notification, it will be "priority=-2"
      When it is idle, it will be "priority=0"

Change-Id: I9ae286d7a2dcc71b059f4521ded451ec4732af18