Introduce a build_android step (#38848)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38848

In order to parallelize the Android CI, I've moved most of the building to a `build_android` step which executes before the `build_npm_package` step.

As currently, building Hermes for Android is on the critical path, this should reduce much of the execution time on CI.

Changelog:
[Internal] [Changed] - Introduce a build_android step

Reviewed By: mdvacca

Differential Revision: D48148418

fbshipit-source-id: bfe3175fcc11d96e264eb31d8d5555bd1f83c01d