move constants in generated android/build.gradle

[brody4hire]

near to the beginning of android/build.gradle

(using single quotes in definition of DEFAULT_BUILD_TOOLS_VERSION)

and remove some comments that are not needed

as a followup to PR #135

verified that:

• ✅ npm t (npm test) passes
• ✅ able to generate view module with example that works with React Native 0.59
• ✅ able to generate non-view module with example that works with React Native 0.59
• ✅ able to generate non-view module with example that works with React Native 0.60
• ✅ able to generate non-view module with example that works with React Native 0.61-rc
• ✅ Travis CI build is green

The one potential drawback is that this update continues with a minor divergence from the React Native template.

/cc @friederbluemle