Add support for -apiMapping parameter in DocLava.

This is used to output a mapping of class member signature to source
position, which in turn is used for automated addition of annotations in
frameworks code.

Metalava support does not exist (yet), and may not be needed, since the
addition of these annotations in a one-time thing. This change can be
reverted once this is complete.

See go/hidden-api-annotations for more context.

Test: m
Bug: 110868826
Change-Id: I90e409f6ea02b16eb9e03d92382f9bb46a8fdab5