license_checker: Add MPL 2.0 "Source Code Form" license file MPL 2.0 Exhibit A indicates that it is acceptable to include the short "Source Code Form" license: https://www.mozilla.org/en-US/MPL/2.0/#exhibit-a---source-code-form-license-notice This allows projects which uses this format to be properly detected as MPL 2.0. Test: ./external/rust/android-crates-io/crate_tool import serialport 4.7.0 Bug: 396413216 Change-Id: I41bad284cd43fcc4c38d93bdb7443b30b8926fb6