Deprecate private AsciiCapable protocol

The concept "AsciiCapable InputMethodSubtype" was initially introduced
as a private protocol based on a magic keyword "AsciiCapable"
specified in "imeSubtypeExtraValue" attribute in API level 15 [1],
then became a public API "isAsciiCapable" attribute in API 19 [2].

However, it turns out that there remains one place in InputMethodUtils
where the previous private protocol is still used.

With this CL, InputMethodUtils stop relying on the previous private
AsciiCapable.

 [1]: I1a83b227498073c47567f73566043c273809adc9
      c3690567
 [2]: Ic3ace4b6e0432d56696bcbc0be336aec1dc744a5
      dc8abf6c

Fix: 78537996
Test: make doc-comment-check-docs -j
Test: atest InputMethodPreferenceTest InputMethodUtilsTest
Change-Id: I56c0c19878657a41882c2d784e1ac96a52ab33f6