Fix red tree (Android module) (flutter#26270)

Author: dnfield

packages/flutter_tools/templates/module/android/host_app_common/app.tmpl/build.gradle.tmpl

@@ -4,6 +4,12 @@ apply plugin: 'com.android.application'
 
 android {
     compileSdkVersion 27
+
+    compileOptions {
+        sourceCompatibility 1.8
+        targetCompatibility 1.8
+    }
+
     defaultConfig {
         applicationId "{{androidIdentifier}}.host"
         minSdkVersion 16