Cleanup IME token validation in IMMS

This cleans up the usages of the IME token in IMMS, ensure this is only
validated once, at the outermost boundary of the flow.

This also adds missing token validation in three stylus methods.

Flag: EXEMPT refactor
Test: atest and checking logcat
Bug: 355154964
Change-Id: I6ca8cd8e4a18412c20b0fec58abc534aa32b70dc