Remove "experimental" prefix from concurrent multi-user IME mode

Now that we have achieved the minimum demoable milestone, we no longer
need to call it experimental. Let's remove such a prefix to make it
clear that the feature development is on track.

This is a mechanical renaming CL. There must be no change in the final
artifacts.

Fix: 351045624
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I7f622e6f7b2125574d7de8c0ecc93b75e7a1b326