[rust] Add SourceProviders as crates support.

This allows SourceProvider modules to create rust_library variants so
that generated source can be referenced as an external crate rather than
via an include macro. This is done by including rust_bindgen modules
like any other library, as a dependency in either rlibs, dylibs, or
rustlibs.

This renames the stem and flags properties for rust_bindgen modules to
source_stem and bindgen_flags, respectively. This deconflicts with the
usage in baseCompiler.

This also removes 'subName' from the Module struct and moves it over to
SourceProvider, which was the only user. This allows us to set it in
baseSourceProvider's AndroidMk; setting it in Module's AndroidMk was
causing problems finding NOTICE files for bindgen library variants.

Bug: 159064919
Test: New Soong tests pass.
Test: Local test rust_binary can use rust_bindgen module as a crate.

Change-Id: Ieb2cb614c2dd0b5aa7120541d77f6f822a6a1806