Do not account recents activity as previous process

When using 3rd party home, the RecentsActivity will be involved in
gesture navigation. E.g. when swiping to home, both the closing app
and recents will be stopped to update previous process.

Because RecentsActivity should already keep a higher oom-adj (100)
by the service binding from systemui to its TouchInteractionService,
it's better to let the real last used app be the previous process.

Also optimize isRecentsComponent a bit because uid check is faster
and easier to skip unnecessary string comparison.

Fix: 415340877
Flag: EXEMPT bugfix
Test: ActivityTaskManagerServiceTests#testUpdatePreviousProcess
Test: Set 3rd party launcher as default home.
      Use swipe-up gesture to return from an app to home.
      "adb shell dumpsys activity o | grep mPreviousProcess"
      The output should show the app.
Change-Id: I88cbb22319f3486a7456d39b7820a4cb9423bc34