Implement "forceAscii" private IME option

This change also introduces "AsciiCapable" extra value for subtype
that indicates the subtype can input any ASCII code point from its
keyboard layout.

Only if the input field has "forceAscii" private IME option and the
current subtype doesn't have "AsciiCapable" extra value, the en_US
keyboard layout will be used for the input field.

Bug: 3384942
Change-Id: I25e2553e37ecb5002df1164e45f6273845fe463b