Add missing Android-changed comments to Bidi.java

Add missing Android-{changed|added} annotations where appropriate and
comment the general style of changes to this class in an Android-note.

This also contains a small code change to the Bidi(String, int)
constructor to align the code closer with upstream OpenJDK 8u121-b13.

Bug: 65669355
Test: CtsLibcoreTestCases
Change-Id: I1ff4db676efbcff476be226b9b0b77bf22573139
1 file changed