add basic signature in template (facebook#22665)

Summary:
react-native-community/cli#38.
add basic signature in template.
Signature file is the same as RNTester.
pass all current ci.
 [Android] [Feature] - add basic signature in template
Pull Request resolved: facebook#22665

Differential Revision: D13490480

Pulled By: hramos

fbshipit-source-id: 9d0ca4debde2ddbde0d4a611b9b41fc24235314a

Author: gengjiawen

template/android/app/build.gradle

@@ -112,8 +112,22 @@ android {
             include "armeabi-v7a", "x86", "arm64-v8a"
         }
     }
+    signingConfigs {
+        debug {
+            storeFile file('debug.keystore')
+            storePassword 'android'
+            keyAlias 'androiddebugkey'
+            keyPassword 'android'
+        }
+    }
     buildTypes {
+        debug {
+            signingConfig signingConfigs.debug
+        }
         release {
+            // Caution! In production, you need to generate your own keystore file.
+            // see https://facebook.github.io/react-native/docs/signed-apk-android.
+            signingConfig signingConfigs.debug
             minifyEnabled enableProguardInReleaseBuilds
             proguardFiles getDefaultProguardFile("proguard-android.txt"), "proguard-rules.pro"
         }