Revert "Conditionally initialize IMMS#mCurrentUserId with a main user"

This reverts commit cd4a8a11 [1].

 [1]: Ie2ee161bea771c5928b504bee494813ac8e1fb7b

Reason for revert:
Turns out that this approach does not work as we expected.
We need to find a different approach.

Bug: 341558132
Flag: android.view.inputmethod.concurrent_input_methods
Test: presubmit
Test: atest FrameworksInputMethodSystemServerTests
Change-Id: Ic8758c99854faba95b00b78dea836aea4ab6bebe