Allow setting a different Gradle plugin version.

Turns out, the Gradle version (2.10) is NOT the same as the plugin version (2.1.0). These have to be treated separately.

dependencies/extension-api/build.gradle

@@ -6,7 +6,7 @@ buildscript {
 	}
 	
 	dependencies {
-		classpath 'com.android.tools.build:gradle:2.1.0'
+		classpath 'com.android.tools.build:gradle:::ANDROID_GRADLE_PLUGIN::'
 	}
 }