Improve handling of certain keyboard layout properties.

Automatically choose a default value for the 'number' property
based on the characters that the key can generate.

Don't generate any character when ctrl, alt or meta is
pressed unless the behavior exactly matches the modifier keys
that are pressed.

Simplified the basic keyboard layouts taking into account the
new features.

Bug: 6110399
Change-Id: Ibc0f0b50c2dcf3f962a33ac77c24d2993b77637d