Support @RequiresFeature, -sdkvalues, etc

This CL includes a number of changes to metalava:

- Track recent features added to doclava, such as
  (a) supporting the @RequiresFeature annotation to insert special
      documentation comments which links to the required feature and
      how to check for it
  (b) omitting "final" from methods in signatures where the
      surrounding class is also final

- Changing the way relative references are handled in javadocs.
  Previously, metalava attempted to rewrite all javadocs to use fully
  qualified references in all cases, such that running javadoc on the
  stubs themselves would work (and since the rest of the stubs are
  using fully qualified references). However, the javadoc rewriting
  was a bit brittle, so instead leave the docs alone and include all
  the import statements from the original code instead (limited to the
  imports for classes/methods that are part of the API that is.)

- Add support for the -sdkvalues flag from doclava1: this basically
  ports the code which lets metalva emit the various SDK files in
  platforms/android-X/data/ such as broadcast_actions.txt and
  widgets.txt.

- Fixing the markdown formatting to correctly handle tables in
  gitiles

- Allow for all compatibility flags to be specified from the command
  line

Test: Unit tests included & updated
Change-Id: Idb13fe42a746cfeebabf65a3c4b4c912fbd22e0e