Make Trampoline support multi-user backup.

We create a differnte suppressBackup file for each user, and gate
operations on that.
Backup still can't be toggled for other users that is not SYSTEM.

Test: atest TrampolineTest
Test: Verify backup works and restore works, just for user 0 for now.
BUG: 121198006

Change-Id: Ib4ecf48d248319e9d890515937fc087ef8f46039