Move IME related seach landing pages to its fragments

This CL moves the code building index of IMEs from
InputMethodAndLanguageSettings to VirtualKeyboardFragment and
AvailableVirtualKeyboardFragment, so that a search landing page of IME
becomes one of the fragment correctly depending on the IME state.

Bug: 32643633
Test: Manually installing/uninstalling AOSP LatinIME.apk while
      Settings app is/isn't running, then search AOSP and verify the
      landing page is correctly determined by the IME's enable state.
Test: Update robolectric DynamicIndexableContentMonitorTest
Change-Id: Ie44f358c2d3919fc864f4f88e6229293113823dd