gradle: add ndk version

Change-Id: Ib2d5e1135dbf7a8c14fb8a390f879ed9dc1e330c
diff --git a/ring-android/app/build.gradle b/ring-android/app/build.gradle
index 1d3fd8f..8b2a606 100644
--- a/ring-android/app/build.gradle
+++ b/ring-android/app/build.gradle
@@ -5,6 +5,7 @@
 android {
     compileSdkVersion 29
     buildToolsVersion '29.0.2'
+    ndkVersion "21.0.6113669"
     defaultConfig {
         minSdkVersion 21
         targetSdkVersion 29
@@ -69,8 +70,8 @@
 dependencies {
     def android_support_core_version = '1.1.0'
     def android_support_version = '1.0.0'
-    def material_version = '1.1.0-rc01'
-    def butterknife_version = '10.2.0'
+    def material_version = '1.1.0-rc02'
+    def butterknife_version = '10.2.1'
     def dagger_version = '2.24'
 
     implementation fileTree(include: '*.jar', dir: 'libs')