fix: add missed external build scripts for android

Author: peilinok

android/build.gradle

@@ -1,6 +1,10 @@
 group = "io.agora.agorartm.agora_rtm"
 version = "1.0"
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 buildscript {
     repositories {
         google()
@@ -26,16 +30,23 @@ android {
         namespace = "io.agora.agorartm.agora_rtm"
     }
 
-    compileSdk = 34
+    compileSdkVersion safeExtGet('compileSdkVersion', 31)
 
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_1_8
         targetCompatibility = JavaVersion.VERSION_1_8
     }
 
     defaultConfig {
+        minSdkVersion safeExtGet('minSdkVersion', 21)
         minSdk = 16
         consumerProguardFiles 'consumer-rules.pro'
+
+        externalNativeBuild {
+            cmake {
+                arguments '-DANDROID_STL=c++_static'
+            }
+        }
     }
 
     sourceSets {
@@ -45,6 +56,12 @@ android {
         }
     }
 
+    externalNativeBuild {
+        cmake {
+            path '../src/CMakeLists.txt'
+        }
+    }
+
     dependencies {
         testImplementation("junit:junit:4.13.2")
         testImplementation("org.mockito:mockito-core:5.0.0")