[...] followup to android build.gradle template setup updates

[brody2consult]

some followup items to PR #135:

• update the generated .npmignore and the files entry as proposed by PRs linked by Add gradle plugin and androidX versions into project-level gradle react-native-community/discussions-and-proposals#151 (comment) & Update build.gradle facebook/react-native#25569:
  • react-native-community/react-native-localize#69
  • react-native-community/react-native-share#588
  • react-native-community/async-storage#209
  • FormidableLabs/react-native-app-auth#384
• move some definitions and imports like I proposed in PRs move constants in generated android/build.gradle #140, move imports in generated android/build.gradle #141, and [...] refactor stand-alone build definitions in android/build.gradle (generated artifact) #142
• add some more comments like I suggested in my comments on pr Update android build.gradle template setup #135
• raise a new issue that describes the main solution from Add gradle plugin and androidX versions into project-level gradle react-native-community/discussions-and-proposals#151 (comment) & Update build.gradle facebook/react-native#25569, without all of the noise
• raise separate issues with recommendations to update the generated .npmignore and the files entry as proposed by the PRs listed above
• update the template in react-native according to the change proposed in PR [wip (tbd)] fix react-native reference comments in build.gradle #154
• consider merging an update similar to (DRAFT) PR [wip (tbd)] fix react-native reference comments in build.gradle #154
• check for any more updates wanted from React Native 0.61 (ref: PR [WIP] Android build.gradle updates from 0.61-stable #107) & 0.62 (RC)

/cc @friederbluemle @SaeedZhiany