Add support for sdk extensions in prebuilt_apis

This makes it possible to pass an extensions_dir containing finalized
module APIs to prebuilt_apis. The extension versions are compared to the
api level versions to figure out what the "latest" finalized API is for
each module. This is done using the base_sdk_extension_version, such
that any extension higher than than base_sdk_extension_version is
assumed to be finalized after any of the existing api level versions.

Bug: 220086085
Test: prebuilt_apis_test.go
Test: existing module in prebuilts/sdk
Change-Id: Ib792f84202d436f594ba5e8716c6a187f9cd60dc