Use CharSequence instead of String in CursorAnchorInfo

This might be useful if we need to pass some spans from the
application to the input method through CursorAnchorInfo.

This change makes CursorAnchorInfo more consistent with other
input method APIs where CharSequence is preferred over String.

BUG: 15088391
Change-Id: Ie04e8a27050849fee1c93ef14b9944acc70fafe4