Remove expensive queries for lock task mode (screen pinning)

- Instead use the TaskStackListener callbacks to update the state
  in each component as needed

Bug: 224544468
Test: Enable/disable lock task mode and
      dumpsys activity service SystemUIService to confirm the sysui state
      is still updated
Test: atest SystemUITests
Change-Id: I15ec6251c75a29c563820ea6424768fa33245497