Remove mCurrentUserId dependency from switchToInputMethodLocked

This is a follow up CL to our previous CL [1], where we started using
the same implementation as the current user in the single-user mode in

  adb shell ime --user <userId> set <imeId>

when IMMS#mConcurrentMultiUserModeEnabled is set.

With this CL we expand the above new behavior unconditionally as part
of our on-going effort to remove the remaining dependency on

  InputMethodManagerService#mCurrentUserId.

 [1]: I58edae672be629bdacd4a9a16cc9ad5c8012eea6
      22992d58

Bug: 350386877
Bug: 350834650
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I57868f616b232a70fec2e3bc2d087f7e5a06f9e6