Consolidate CI/local test scripts

Summary:
Update `scripts/run-android-emulator.sh` to use same settings as Circle CI when creating and launching an AVD. This helps provide a known good configuration, useful for running Android tests locally.

Run `scripts/run-android-emulator.sh` and confirm AVD is created && launched
Run `scripts/run-android-emulator.sh` again and see message about AVD already running
Run `test_android` on CI and observe AVD is launched.

Run `runXcodeTests.sh` and confirm unit tests run.
Run CI and confirm iOS unit tests run.

[GENERAL][MINOR][`scripts`] - Consolidate CI/local test scripts
Closes https://github.com/facebook/react-native/pull/19228

Differential Revision: D8019888

Pulled By: hramos

fbshipit-source-id: 28b12c2e781ee80bcc90c22e691a5acb16232369