Fixes issue with finding React Native version

* Fixes 284

Searching for reactNative version in projects ext root now uses safeExtGet to stop build issues where reactNative is not set.

android/build.gradle

 
 dependencies {
     implementation "com.android.support:support-v4:${safeExtGet('supportLibVersion', '27.+')}"
-    api "com.facebook.react:react-native:${rootProject.ext.reactNative}"
+    api "com.facebook.react:react-native:${safeExtGet('reactNative', '+')}"
 }