Using SDK Version variables from root project

Instead of assuming the `compileSdkVersion`, `targetSdkVersion`, etc, read it from the root project.
Default `compileSdkVersion` and `targetSdkVersion` to the latest versions.

Android Target API Level 26 will be required in August 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html
And the React Native team is already working on this:
facebook/react-native#17741
facebook/react-native#18095

android-exoplayer/build.gradle

@@ -1,17 +1,28 @@
 apply plugin: 'com.android.library'
 
+def _ext = rootProject.ext
+
+def _reactNativeVersion = _ext.has('reactNative') ? _ext.reactNative : '+'
+def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 27
+def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '27.0.3'
+def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
+def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 27
+
 android {
- compileSdkVersion 23
- buildToolsVersion "25.0.2"
+ compileSdkVersion _compileSdkVersion
+ buildToolsVersion _buildToolsVersion
 
  defaultConfig {
- minSdkVersion 16
- targetSdkVersion 23
+ minSdkVersion _minSdkVersion
+ targetSdkVersion _targetSdkVersion
+ versionCode 1
+ versionName "1.0"
  }
 }
 
 dependencies {
- provided 'com.facebook.react:react-native:+'
+ //noinspection GradleDynamicVersion
+ provided "com.facebook.react:react-native:${_reactNativeVersion}"
  compile 'com.google.android.exoplayer:exoplayer:r2.4.0'
  compile('com.google.android.exoplayer:extension-okhttp:r2.4.0') {
  exclude group: 'com.squareup.okhttp3', module: 'okhttp'

android/build.gradle

@@ -1,12 +1,20 @@
 apply plugin: 'com.android.library'
 
+def _ext = rootProject.ext
+
+def _reactNativeVersion = _ext.has('reactNative') ? _ext.reactNative : '+'
+def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 27
+def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '27.0.3'
+def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
+def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 27
+
 android {
- compileSdkVersion 25
- buildToolsVersion "25.0.2"
+ compileSdkVersion _compileSdkVersion
+ buildToolsVersion _buildToolsVersion
 
  defaultConfig {
- minSdkVersion 16
- targetSdkVersion 25
+ minSdkVersion _minSdkVersion
+ targetSdkVersion _targetSdkVersion
  versionCode 1
  versionName "1.0"
  ndk {
@@ -17,6 +25,6 @@ android {
 
 dependencies {
  //noinspection GradleDynamicVersion
- provided 'com.facebook.react:react-native:+'
+ provided "com.facebook.react:react-native:${_reactNativeVersion}"
  compile 'com.yqritc:android-scalablevideoview:1.0.4'
 }