upgrade gradle file: remove deprecated buildToolsVersion, use saftExtGet for react-native version

Author: Marcel Kalveram

android/build.gradle

@@ -7,7 +7,6 @@ def safeExtGet(prop, fallback) {
 
 android {
  compileSdkVersion safeExtGet('compileSdkVersion', 28)
- buildToolsVersion safeExtGet('buildToolsVersion', "28.0.3")
 
  defaultConfig {
  minSdkVersion safeExtGet('minSdkVersion', 19)
@@ -24,6 +23,6 @@ android {
 }
 
 dependencies {
- implementation 'com.facebook.react:react-native:+'
+ implementation "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
 }