Blind fix for user switcher instant crash.

There is an instant crash stemming from a premature optimization done in
the newly refactored code for the user switcher system.

I am unable to reproduce this locally, so this is a blind fix. What
follows is the rationale behind it.

It appears to be a race condition.
UserRepositoryImpl#isSimpleUserSwitcher asserts that we have loaded the
settings but the settings are only loaded in a background coroutine job
that's kicked off when the repository class is instantiated. From the
looks of it, the former path is triggered before the latter is.

The fix, therefore, is to always make sure that there is a non-null
value for user settings. This is achieved in this CL using an expensive
runBlocking call at class instantiation time. Delaying that was an
optimization that attempted to do better than the original pre-refactor
code, where the same approach is taken (settings are accessed on the
main thread in a blocking fashion).

Fix: 255916597, 254764031
Test: unable to reproduce locally but made sure that, with the new
changes, there is no instant crash nor a crash when entering the
full-screen user switcher on one of the affected device models - used
adb reboot to trigger restarts.

Change-Id: I970a416ae7eaf2d2a366b12b3d7c6b98ebc52011