enhance RNTester android config

Summary:
* cliPath is not config right
* root config can be better config instead of a relative path which can easily go wrong if location changed
* DeveloperSupport should only be in debug mode.

* make https://github.com/facebook/react-native/pull/18732 change on local.
* config signingConfig
* execute `./gradlew :RNTester:android:app:assembleRelease`, and run app on device to check everything is fine.

none

[GENERAL][ENHANCEMENT][RNTester]
Closes https://github.com/facebook/react-native/pull/18746

Differential Revision: D7548846

Pulled By: hramos

fbshipit-source-id: 8943f84a6c99456477dff2deeaacc96f093b2e09