Add an overlayable config for IME switcher visibility.

Seems that there are two mutually exclusive requests about how IME
switcher visibility should be controlled.
  A. Requests like Bug 19496012.  We should show the IME switcher
     as a quick access to "Show input method" setting when a physical
     keyboard is attached via wireless connections that do not have
     clear connection/disconnection affordance (e.g. Bluetooth
     keyboards).
  B. Requests like Bug 25432652.  We should not have a rule like A
     when a physical keyboard is attached with clear
     connection/disconnection affordance (e.g. USB wired keyboards,
     2-in-1 convertible tables w/ magnetic contacts).

Currently satisfying both requests at the same time is really difficult
because InputDevice does not have such an attribute.  Even with such an
attribute, it's still an open question about how to deal with two or
more keyboards.  As a short term solution, this CL add an overlayable
config so that each device can configure which strategy to apply as the
default behavior.

Bug: 26245853
Change-Id: Id2aef6597916422ea63435ae9c31a9a9b5ddf5b8