java_sdk_library: Allow dist artifacts to be named

Unless no_dist: true is specified the java_sdk_library will
automatically copy the stubs jar and the API specification file (but
not the removed API specification file) for each scope to the apistubs/
directory in the dist from which the sub-directories in prebuilts/sdk
are populated. Previously, the name of those artifacts was based on the
name of the module. This change adds the dist_stem property to allow
the name of those artifacts to be specified separately to the module.

The purpose of this is to allow the art.module.public.api,
conscrypt.module.public.api and i18n.module.public.api java_sdk_library
modules to use a different name when copying to the dist,
e.g. conscrypt.module.public.api should use conscrypt.

Additionally, as the artifacts in the dist end up being copied verbatim
to prebuilts/sdk and the prebuilt_apis module uses those names to
create a filegroup referencing the latest released version of the API
and removed API specification files the dist_stem property is also
used in constructing the name of those filegroups.

Bug: 173715943
Test: lunch sdk-eng && m dist sdk
      Compare the results of the above before and after the change to
      ensure that this change does not affect the contents of the dist.
      See the topic's other change in external/conscrypt for testing
      the behavior of this new property.
Change-Id: Ie81345021991ff91ca55fec15b02627135293308