little improvement over too many "def"

just added a function which does acts like a ternary with fallback option. Hence, less cluttered "def" variables  
additionally, changes the SDK values from 23 to 26 as per new changes from react-native and Android

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#18095
facebook/react-native#17741

android/build.gradle

@@ -9,24 +9,19 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
-def _ext = rootProject.ext
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
-def _reactNativeVersion = _ext.has('reactNative') ? _ext.reactNative : '+'
-def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 23
-def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '23.0.1'
-def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
-def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 22
-def _glideVersion = _ext.has('glideVersion') ? _ext.glideVersion : '4.7.1'
-def _excludeAppGlideModule = _ext.has('excludeAppGlideModule') ? _ext.excludeAppGlideModule : false
 
 android {
-    compileSdkVersion _compileSdkVersion
-    buildToolsVersion _buildToolsVersion
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
 
     sourceSets {
         main {
             java {
-                if (_excludeAppGlideModule) {
+                if (safeExtGet('excludeAppGlideModule', false)) {
                     srcDir 'src'
                     exclude '**/FastImageGlideModule.java'
                 }
@@ -35,8 +30,8 @@ android {
     }
 
     defaultConfig {
-        minSdkVersion _minSdkVersion
-        targetSdkVersion _targetSdkVersion
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
     }
@@ -54,16 +49,16 @@ repositories {
 
 dependencies {
     //noinspection GradleDynamicVersion
-    compile "com.facebook.react:react-native:${_reactNativeVersion}"
-    compile "com.android.support:support-v4:${_compileSdkVersion}"
-    compile("com.github.bumptech.glide:glide:${_glideVersion}") {
+    compile "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
+    compile "com.android.support:support-v4:${safeExtGet('compileSdkVersion', 26)}"
+    compile("com.github.bumptech.glide:glide:${safeExtGet('glideVersion', 4.7.1)}") {
         exclude group: "com.android.support"
     }
-    compile("com.github.bumptech.glide:annotations:${_glideVersion}") {
+    compile("com.github.bumptech.glide:annotations:${safeExtGet('glideVersion', 4.7.1)}") {
         exclude group: "com.android.support"
     }
-    annotationProcessor "com.github.bumptech.glide:compiler:${_glideVersion}"
-    compile("com.github.bumptech.glide:okhttp3-integration:${_glideVersion}") {
+    annotationProcessor "com.github.bumptech.glide:compiler:${safeExtGet('glideVersion', 4.7.1)}"
+    compile("com.github.bumptech.glide:okhttp3-integration:${safeExtGet('glideVersion', 4.7.1)}") {
         exclude group: "com.android.support"
         exclude group: 'glide-parent'
     }