Use default keyboard layout by default

Most ibus-m17n engines should not change the keyboard layout

Unless there is a good reason to change the keyboard layout,
most ibus-m17n engines should just keep the current keyboard layout.

Currently, selecting engines like latn-post, latn-pre, da-post,
... switches to US English keyboard layout.  But this is not nice
because these input methods can be used with almost any keyboard
layout (as long as it can be used to input ASCII).

For example, a user who uses German keyboard layout and wants to use
the latn-post input method to type other Latin based languages, will
certainly want to keep the German layout while using latn-post.

BUG=

Review URL: https://codereview.appspot.com/7323074
Patch from Mike FABIAN <maiku.fabian@gmail.com>.