Remove unintentinal early-returns in IMMS

This is a follow up CL to my previous CLs [1][2], which aimed to
implement a per-user initialization logic for concurrent multi-user
mode.

Apparently there are questionable early-returns in

  #experimentalInitializeVisibleBackgroundUserLocked().

Let's make sure that the entire initialization logic is executed by
removing them.

There must be no behavior change in the existing single user mode, and
for concurrent multi-user mode, the new behavior should be the right
one.

 [1]: I82cfc366b614d7e55a0f4fa11d5eaf5f0f8eb0a8
      1ec914a0
 [2]: I414a228e068fa53223234fe1db3e813474841604
      f7d90f36

Bug: 341199701
Bug: 341558132
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: If659c309d57bf498aa5614734a6409ce478b7d8a