Have both NAVIGATION_HINT_IME{,_SWITCHER}_SHOWN

This is a mechanical renaming CL with no behavior change.

Somehow NAVIGATION_HINT_IME_SHOWN was set when the IME switcher needs
to be shown, not when the IME is shown.  This CL aims to avoid future
confusions by also introducing

  NAVIGATION_HINT_IME_SWITCHER_SHOWN

then setting NAVIGATION_HINT_IME_SHOWN when the IME is actually shown.

This CL also updates NavigationBarTest to verify those flags.

Bug: 192412909
Bug: 217755059
Test: atest SystemUITests:NavigationBarTest
Change-Id: I8bdd58d040689953072c7f384ece2bca677265e1