Introduce MultiUserUnawareField annotation as a hint

In order to keep track of our progress on Bug 305849394, this CL
introduces a no-op marker annotation

  MultiUserUnawareField

to indicate which field is not yet ready for concurrent multi-user
mode in InputMethodManagerService.

This is purely for informative purpose and should have no impact at
the run time.

Bug: 305849394
Test: presubmit
Change-Id: I3f0e1af357dc6fbe13139c9fe32e968b20580815