Use boolean flag for showing IME picker

In [1] we moved all invocations of InputMethodMenuControllerNew to mNewInputMethodSwitcherMenuEnabled instead of checking the feature flag, to also disable it on watches and small screens. However, this resulted in a logical merge conflict in [2] which still uses the feature flag.

  [1]: I19d46f4351f8f52636d622b433c2c15e77b77953
  [2]: I0963968ad2a94144178d72f3d28ee8d31ac4f46e

Change-Id: I8030d69c31572d3886bf2db7e695776ea54de3f6
Flag: android.view.inputmethod.ime_switcher_revamp
Test: presubmit
Bug: 311791923
Fix: 355758678