[Android]fix gradle problem.

Author: xgfd3

Android/APIExample-Audio/app/build.gradle

@@ -8,7 +8,7 @@ android {
 
     defaultConfig {
         applicationId "io.agora.api.example"
-        minSdkVersion 19
+        minSdkVersion 21
         targetSdkVersion 32
         versionCode 1
         versionName "1.0"