Simplify boilerplate templates

[gregswindle]

User story

As a contributor,
I need to clearly understand generator-android-lib
In order to maintain and extend it.

Acceptance criteria

• 1. Define dependency versions in a single place
• 2. build.gradle files conform to the SRP (Single Responsibility Principle)

Expected Behavior

1. Versions are defined in an ext.
2. Responsibilities are packaged semantically, e.g., lint and code smellers are bundled in a quality-gate.gradle file.

Current Behavior

1. Versions are bound to their dependencies
2. Concerns and responsibilities are "scattered" among different *.gradle files, e.g., static-code analysis functions a parent build.gradle file.

Possible Solution

Context