Bug 913782 - [Keyboard] Build time default keyboard selection work for
3rd-party keyboard support
 - make build/settings.js read the default keyboard layout config

Bug 920431 - [keyboard] keyboard_layout.json should be customizable
  - Refine to make it adopt the config in distribution package
Merge pull request #15410 from RudyLu/keyboard/Bug920431-keyboard_layout_customize

Bug 920431 - [keyboard] keyboard_layout.json should be customizable.
r=yurenju