Move physical keyboard search landing page to its Fragment

This CL moves the code building index of physical keyboards from
InputMethodAndLanguageSettings to PhysicalKeyboardFragment, so that a
search landing page becomes PhysicalKeyboardFragment as expected.

Bug: 32643633
Test: Connecting/Disconnecting Anker bluetooth keyboard while Settings
      app is/isn't running, the search Anker and verify landing page
      is PhysicalKeyboardFragment.
Test: Update robolectric DynamicIndexableContentMonitorTest.

Change-Id: I51f308ada74966a96c258f8ef50a9b4fc7bc6635