Update Gradle configs

Configure project-wide properties.
https://developer.android.com/studio/build/gradle-tips#configure-project-wide-properties

android/app/build.gradle

@@ -91,13 +91,13 @@ def enableSeparateBuildPerCPUArchitecture = false
 def enableProguardInReleaseBuilds = false
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion rootProject.ext.compileSdkVersion
+    buildToolsVersion rootProject.ext.buildToolsVersion
 
     defaultConfig {
         applicationId "com.zulipmobile"
-        minSdkVersion 19
-        targetSdkVersion 22
+        minSdkVersion rootProject.ext.minSdkVersion
+        targetSdkVersion rootProject.ext.targetSdkVersion
         versionCode 13
         versionName "1.0.13"
         ndk {
@@ -176,7 +176,7 @@ dependencies {
     compile project(':react-native-device-info')
     compile project(':react-native-vector-icons')
     compile fileTree(dir: "libs", include: ["*.jar"])
-    compile "com.android.support:appcompat-v7:23.0.1"
+    compile "com.android.support:appcompat-v7:${rootProject.ext.supportLibVersion}"
     compile "me.leolin:ShortcutBadger:1.1.16@aar"
     compile "com.facebook.react:react-native:+"  // From node_modules
     compile 'com.facebook.fresco:animated-gif:1.3.0' // For animated GIF support

android/build.gradle

@@ -3,9 +3,13 @@
 buildscript {
     repositories {
         jcenter()
+        maven {
+            url 'https://maven.google.com/'
+            name 'Google'
+        }
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.2.+'
+        classpath 'com.android.tools.build:gradle:2.3.3'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
@@ -20,5 +24,17 @@ allprojects {
             // All of React Native (JS, Obj-C sources, Android binaries) is installed from npm
             url "$rootDir/../node_modules/react-native/android"
         }
+        maven {
+            url 'https://maven.google.com/'
+            name 'Google'
+        }
     }
 }
+
+ext {
+    buildToolsVersion = "26.0.3"
+    minSdkVersion = 19
+    compileSdkVersion = 26
+    targetSdkVersion = 26
+    supportLibVersion = "26.1.0"
+}