[Android] Use an appropriate Android support library version

It must match the compileSdkVersion major number.

android/build.gradle

     repositories {
         mavenLocal()
         jcenter()
+        maven {
+            // Google's maven repository, required for AppCompat
+            url "https://maven.google.com"
+        }
         maven {
             // All of React Native (JS, Obj-C sources, Android binaries) is
             // installed from npm.

android/sdk/build.gradle

 dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
 
-    compile 'com.android.support:appcompat-v7:23.0.1'
+    compile 'com.android.support:appcompat-v7:25.4.0'
     compile 'com.facebook.react:react-native:+'
 
     compile project(':react-native-background-timer')