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/build.gradle b/build.gradle
index 68cc5a0..cea6a64 100644
--- a/build.gradle
+++ b/build.gradle
@@ -19,18 +19,21 @@
 import groovy.io.FileType
 
 buildscript {
+    apply from: 'buildSrc/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.
-        classpath 'com.android.tools.build:gradle:2.2.4'
+        classpath libs.gradle
     }
 }
 
+apply from: 'buildSrc/dependencies.gradle'
+
 repositories {
     maven { url '../../prebuilts/tools/common/m2/repository' }
 }
@@ -43,8 +46,8 @@
 dependencies {
     doclava project(':doclava')
     jdiff project(':jdiff')
-    jdiff 'xerces:xmlParserAPIs:2.6.2'
-    jdiff 'xerces:xercesImpl:2.6.2'
+    jdiff libs.xml_parser_apis
+    jdiff libs.xerces_impl
 }
 
 // Version code components.
@@ -53,10 +56,6 @@
 // This number gets incremented for each public release.
 ext.extraVersion = 41
 
-// Dependency versions.
-ext.testRunnerVersion = '0.6-alpha'
-ext.espressoVersion = '2.3-alpha'
-
 // Enforce the use of prebuilt dependencies in all sub-projects. This is
 // required for the doclava dependency.
 ext.usePrebuilts = "true"