Ignore comment lines in hidden api lists

It is sometimes useful to add comments at the top of a source file. This
patch changes hidden API list generation to ignore lines beginning with
a hash.

Note that due to the sorting constraints on hidden API lists, comments
can be only at the top of the files.

Test: m appcompat
Change-Id: I1bc6fd44d1b1f10a5adc45093d7f7ed5a0c5a54f