VDM IME 1/n: isVirtualDeviceOnly attribute

 - Add a feature flag
 - Add a isVirtualDeviceOnly InputMethod attribute to hide custom
   VD IME components from settings. Such components should not be
   visible to users.

Bug: 287269288
Test: atest InputMethodInfoTest
Change-Id: I80422f9002d5c412dfd556966203759d90236db8