Bind userId with ImeBindingState

By binding userId with ImeBindingState, IMMS will be able to know who
exactly owns the currently focused window (some windows may be owned by
system user, therefore is not always possible to find out who is the real
user who owns certain window).

This CL represents an internal refactoring and shouldn't introduce any
observable breakage.

Bug: 314149476
Test: atest FrameworksInputMethodSystemServerTests
Test: atest --host FrameworksInputMethodSystemServerTestsRavenwood
Change-Id: I79e2f6463192a39ef7c6820751b2908b460d9e37