Fix User Switch Timeout Issue

[This is a manual cherrypick of aosp/2898333]

While Switching between Users, UserTracker Callbacks are taking more time which is more than 5 secs (set by AOSP) due to some usertracker callbacks are getting executed on Main Thread which is not guaranteed to execute on time. As we have a synchronization mechanism with CountDownLatch, we can safely execute on Bg Executors

Bug: 319352292
Test: Boot and Check User Switch timeout
Flag: NONE

Change-Id: I0d83bb51b63d49e0a69a143c834572c4192c9f24