Added log statements to some user-switching tasks.

NOTE: as user switch is a very low-frequency operation, these logs statements
don't need to be guard (to avoid spam).

Test: manual verification
Bug: 141502467
Bug: 140999054
Bug: 145027829

Change-Id: I7122e45c45a646419b03242c99d90417a29da92a
(cherry picked from commit ebccffe3b10d8642e7b1d64f183b627f644011a4)