Add droiddoc_template

We prefer not to use absolute paths in modules, but to reference modules
that have associated paths. This a few benefits:

* it's easier to move a module than to update all the references
* if the module doesn't exist, we treat it as a normal missing
dependency, not having to deal with the missing dependency in path.go
* implementing visibility(etc) in the future would be simpler if there
was a module attached to the reference, so we don't have to do various
path-based lookups to try and match things up.

So define a `droiddoc_template` module, which takes a path, and will run
the glob once in that module. All of the `droiddoc` modules can then
specify it through the `custom_template` property, which will pull the
necessary data.

Also fix that htmldirs should be references from the local path, the
htmldir2 argument never being specified, and complain if more than two
htmldirs are specified, or if the custom template isn't specified.

Test: m core-docs
Test: out/soong/build.ninja is nearly identical
      - line numbers in comments
      - adds directories to droiddoc template dependency lists, which
        is more correct, since we need to rerun on added or removed
	files too.
Change-Id: Iff630bddb3818b8eeed439de7e41fc7fbe7cdcb0