Simplify IMMS#showInputMethodMenu() a bit

SwitchingController#getSortedInputMethodAndSubtypeListLocked() is
anyway called to determine what items should be shown on the IME
switcher dialog, whether there is no item to show or not should also
be determined by the same method.  There is no need to call
getExplicitlyOrImplicitlyEnabledInputMethodsAndSubtypeListLocked()
just to check whether its result is empty or not.

Bug: 119837138
Test: Locally verified as follows:
  1. Build and flash aosp_taimen-userdebug into taimen.
  2. adb shell "settings put secure enabled_input_methods 'com.android.inputmethod.latin/.LatinIME;-921088104;-1337596075'"
  3. Open the Dialer app and tap the top edit field.
  4. Long tap the space key on AOSP Keyboard.
  5. Make sure that IME switcher dialog is shown.
  6. make -j SoftKeyboard
  7. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
  8. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
  9. Tap the keyboard icon on the navigation bar.
 10. Make sure that IME switcher dialog is shown.
Change-Id: Ie166bbd86179db1dae9e31f25d45e76c5e1b15ad