Reduce the direct access to mCurTokenDisplayId in IMMS

In order to make the trnsition to make IMMS#mCurTokenDisplayId
per-user easier, this CL machanically replaces a read access to
mCurTokenDisplayId in IMMS with getCurTokenDisplayIdLocked().

This way we only need to update IMMS#getCurTokenDisplayIdLocked() in a
subsequent CL without touching many lines.

This is a mechanical code change. There must be no observable behavior
change.

Bug: 341179600
Test: presubmit
Change-Id: I833c3ebee8dc909c5b6399a62ec089066f0b5f94