Support building crate variants.

The cargo2android.json config file now supports the key "variants" with
a list of entries containing keys and build name suffix. Each entry
will produce a different library (or binary). Keys supplied for each
variant will overwrite any existing ones specified in the config file.

See an example of the protobuf crate on aosp/2368788.

Test: None
Change-Id: Iaca67c7f22718c27b3cb118d9794f1ff01d31c84
1 file changed