Unify systemReady() handling for each user in IMMS

With this CL, per-user initialization after systemReady() in IMMS will
be unified between single-user mode and multi-user mode.

Bug: 350386877
Fix: 353751919
Test: atest ConcurrentMultiSessionImeTest
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I581fd2057ecf2a3165795d3aff817ee5b367b59e