Fix inconsistencies in CursorAnchorInfo#equals

This CL addresses some inconsistencies found in
CursorAnchorInfo#equals.

- #mComposingText is now taken into consideration when
  compared with other instances.
- NaN is now treated as if it was just a number in
  CursorAnchorInfo#equals if and only if NaN is used in
  -- #mInsertionMarkerHorizontal
  -- #mInsertionMarkerTop
  -- #mInsertionMarkerBaseline
  -- #mInsertionMarkerBottom.
  This is useful if we want to use NaN as a marker to
  indicate that no value is specified.

Change-Id: Ibff31bc34b9d488731b294ad38240c567a06b627