Utilize propdeps-plugin in build

This plugin provides a more convenient API for managing dependencies
within a gradle build.

See https://github.com/spring-gradle-plugins/propdeps-plugin

Issue: ANDROID-156
Build test projects against Android 4.4.2 (API level 19)

Utilizing the Maven Android SDK Deployer, the android.jar from the
Android SDK may be deployed to a local Maven repository.

See https://github.com/mosabua/maven-android-sdk-deployer

Issue: ANDROID-157
Add OkHttp ClientHttpRequestFactory implementation

This commit adds OkHttpRequestFactory which can be used as an
alternative to the native HttpUrlConnection or HttpComponents clients
provided by SimpleClientHttpRequestFactory and
HttpComponentsClientHttpRequestFactory.

Issue: ANDROID-138
GitHub: 10, 12