VDM IME 3/n: Respect VD IME in IMMS

 - Write default device IME to settings when we switch away from it
 - Fallback to NO_IME if the VDM custom IME is not available

Adding an overload of setInputMethodLocked because we don't want to
change the default IME if we're currently on a virtual device but we
do want to update the default device IME setting so the correct one
is restored once we move back to the default device.

Bug: 287269288
Test: CTS
Change-Id: Id05de00da4cc6fce284697acaf76237b0c7b7715