[Media ML] Copy over ActivityManager#handleIncomingUser

This CL copies over the implementation of
ActivityManager#handleIncomingUser except for the case of
USER_CURRENT_OR_SELF. See go/replace-handleincominguser Section
"New Implementation" for reasoning behind this.

Bug: 163336589
Test: Manually test USER_CURRENT and USER_ALL cases via
PipMediaController and NotificationMediaManager.

Change-Id: Id6dbf61d1e66c8e20fc9608ec2bc1df0486b92e2