Put dependencies and their versions into one place.

Support library modules were hardcoding versions of each library
each each build.gradle file. This change moves all these to
buildSrc/dependencies.gradle so we can update the version in the
future with a single file change.

Test: ./gradlew clean assemble assembleAndroidTest still works.
Change-Id: I4238c1182c095ae80d1d6dbb751f71c0ddfe191f
diff --git a/buildSrc/build.gradle b/buildSrc/build.gradle
index 0fc5a1e..ae9582a 100644
--- a/buildSrc/build.gradle
+++ b/buildSrc/build.gradle
@@ -1,12 +1,13 @@
 apply plugin: 'groovy'
 
+apply from: "dependencies.gradle"
+
 repositories {
     maven { url '../../../prebuilts/gradle-plugin' }
     maven { url '../../../prebuilts/tools/common/m2/repository' }
     maven { url '../../../prebuilts/tools/common/m2/internal' }
-    maven { url "../../../prebuilts/maven_repo/android" }
+    maven { url '../../../prebuilts/maven_repo/android' }
 }
 dependencies {
-    // Keep gradle plugin version in sync with ub_supportlib-master manifest.
-    compile 'com.android.tools.build:gradle:2.2.4'
+    compile libs.gradle
 }