Add multi-user support for kids mode

Listener to ACTION_USER_SWITCH intent and update the settings key when
user switched.

Bug: 231575466
Test: 1. set different settings keys for different user.
      2. switch between users and see if the setting value is correct.
Change-Id: Ia1afcb6121b296b293bef0ba7d0a0be25b857554
4 files changed