Add ability to override Android Gradle Plugin used.

- Split the logic for adding repositories to repos.gradle
- Fix broken ALLOW_PUBLIC_REPOS
- Add GRADLE_PLUGIN_REPO and GRADLE_PLUGIN_VERSION to allow to
  override the version of android gradle plugin used.

There should be no impact on most developers when they do not specify
these env variables.

Bug: 37288149
Test: 1. check out studio-master-dev-supportlib
      2. GRADLE_PLUGIN_REPO=/path/to/out/repo GRADLE_PLUGIN_VERSION=2.5.0-dev \
         ./tools/gradlew -p frameworks/support/ assembleDebug

Change-Id: I11506a5f173c89667c0ff516d43badf6efc35b29
(cherry picked from commit 57d3865d1ff4c88b6f00f3c4197731303fabd587)
diff --git a/build.gradle b/build.gradle
index 8af1eb8..64a08a2 100644
--- a/build.gradle
+++ b/build.gradle
@@ -16,17 +16,19 @@
 
 buildscript {
     ext.supportRootFolder = project.projectDir
+    apply from: 'buildSrc/repos.gradle'
+
     apply from: 'buildSrc/init.gradle'
     init.loadDefaultVersions()
     init.setSdkInLocalPropertiesFile()
-    init.addMavenRepositories(repositories)
+    repos.addMavenRepositories(repositories)
 
     dependencies {
         classpath libs.gradle
     }
 }
 
-init.addMavenRepositories(repositories)
+repos.addMavenRepositories(repositories)
 
 init.setupRepoOutAndBuildNumber()