Adding flags to filter dumping of activities.

- To make it consistent with other dump calls, by default dumping
  'activity all' or 'activity <activity>' will now dump it if it exists
  even if it is not in a visible stack.  If -v is specified, only
  activities in visible stacks will be dumped, and if -f is specified
  then only activities in the focused stack will be dumped.

Bug: 35850518
Test: adb shell dumpsys activity <package> and ensure it runs in the background
Change-Id: Ib8284aedce8073e2a30023a7cd925c6c525817c8