[Feature] Add release signing for sample.

Author: zhanghai

sample/build.gradle

@@ -14,10 +14,20 @@ android {
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }
+    signingConfigs {
+        release {
+            storeFile file(System.getenv("STORE_FILE") ?: "/dev/null")
+            storePassword System.getenv("STORE_PASSWORD")
+            keyAlias System.getenv("KEY_ALIAS")
+            keyPassword System.getenv("KEY_PASSWORD")
+        }
+    }
     buildTypes {
         release {
             minifyEnabled true
-            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
+            shrinkResources true
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+            signingConfig signingConfigs.release
         }
     }
 }