Specify java version to avoid assembling APK failure

Server-Protocol-Buffer/build.gradle

@@ -5,6 +5,9 @@ sourceSets {
     main.java.srcDirs += "${protobuf.generatedFilesBaseDir}/main/javalite"
 }
 
+sourceCompatibility = "8"
+targetCompatibility = "8"
+
 dependencies {
     implementation fileTree(dir: 'libs', include: ['*.jar'])
     api 'com.google.protobuf:protobuf-lite:3.0.1'