Add mechanism to boost the shell background thread as needed

- This will be used when things offloaded to the BG thread need to
  be boosted in thread priority (ie. when loading icons along
  critical paths)

Bug: 370895085
Flag: EXEMPT bugfix
Test: atest WMShellUnitTests:HandlerExecutorTest
Change-Id: I531922a2f7b993df1ce4723b4b313bb070f728e7