Use correct call to get Android user ID

Rather than trying to store the active user ID, use
multiuser_get_user_id to pull the actual ID for the caller. This will
now behave correctly when called by background processes.

Test: CtsKeystoreTestCases, from both owner and guest users
Bug: 113280004
Change-Id: I73fd2ef2ac4b1ceb7ec3792761e827d801d0d905
3 files changed