[Nix] Add support for Android build on Darwin

Summary:
Provide defaults for openjdk and the ANDROID_SDK_ROOT (follows
android studio defaults).

https://linear.app/comm/issue/ENG-1443/

Test Plan:
On an intel or m1 mac:
Follow existing directions to download android sdk and ndk

nix develop
cd native/android && ./gradlew bundleRelease --no-daemon

Reviewers: varun, atul

Reviewed By: varun, atul

Subscribers: atul, ashoat, tomek, adrian, abosh

Differential Revision: https://phab.comm.dev/D4605