java_sdk_library: Allow separate api/stubs source modules

The API file for a scope represents the differences between the API
provided by that scope and that provided by the scope that it extends.
On the other hand the stubs source for a scope represents the union of
the API provided by the scope and the scope it extends (all the way
back to public).

Unfortunately, while metalava supports this behavior for scopes that
extend public (e.g. system and test) it does not support this behavior
for scopes that extend others, e.g. module_lib which extends system.
This is because it always assumes that the baseline for the API file
is 'public' and so has no way to defined other baselines.

This change works around that by having separate droidstubs modules to
generate the API and stubs sources for scopes that require different
arguments to generate the API and stubs sources.

Test: m checkapi
Bug: 155164730
Change-Id: Iea7d59852d7aeb503120acf3c44e08eb0d9d07b9