Switch and store keyboard layouts based on IME subtype.

Rather than associate the keyboard layout solely with a specific
hardware model, we should also associate it with a given IME subtype.
This lets users switch between various languages and have the
keyboard change in unison with them so they can use the appropriate
layouts for each language.

This change adds initial support for associating IME subtypes and
keyboard layouts. We still need to:
  - Remove support for the old style of layout association once the
    Settings apps begins to use the new APIs
  - Automatically select an appropriate layout based on the given
    subtype (or set a reasonable universal default such as QWERTY)

Bug: 25752812

Change-Id: Ie88ce1ab77dbfe03ab51d89c1dc9e0a7ddbb3216