[native][android] move rustup target add from CMakeLists.txt to build.gradle

Summary: It looks like cargo is trying to use the new cross-compilation targets before they're available in the rust toolchain. If we make them available in a preceding task in build.gradle, we avoid this race condition.

Test Plan: cd native/android && ./gradlew bundleRelease

Reviewers: atul, jon

Reviewed By: atul, jon

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5454