Swap google() and jcenter() in docs and gradle config.

This seems to be more stable in case Bintray has issues updating the ExoPlayer
sources.

Issue:#4997

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=218327350

README.md

@@ -33,8 +33,8 @@ included in the `build.gradle` file in the root of your project:
 
 ```gradle
 repositories {
-    jcenter()
     google()
+    jcenter()
 }
 ```
 

RELEASENOTES.md

@@ -33,6 +33,8 @@
 * Fix issue where a `NullPointerException` is thrown when removing an unprepared
   media source from a `ConcatenatingMediaSource` with the `useLazyPreparation`
   option enabled ([#4986](https://github.com/google/ExoPlayer/issues/4986)).
+* Swap recommended order for google() and jcenter() in gradle config
+  ([#4997](https://github.com/google/ExoPlayer/issues/4997)).
 
 ### 2.9.0 ###
 

build.gradle

@@ -13,8 +13,8 @@
 // limitations under the License.
 buildscript {
     repositories {
-        jcenter()
         google()
+        jcenter()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:3.1.4'
@@ -32,8 +32,8 @@ buildscript {
 }
 allprojects {
     repositories {
-        jcenter()
         google()
+        jcenter()
     }
     project.ext {
         exoplayerPublishEnabled = true