move imports in generated android/build.gradle

[brody4hire]

move the following lines near the beginning:

apply plugin: 'com.android.library'
apply plugin: 'maven'

as a followup to PR #135

verified the following with PR #140 (move some constants) applied to my workarea:

• ✅ 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

/cc @friederbluemle