android-interop-testing: Remove usage of MultiDexApplication

Since we're only supporting API levels 23+, all the supported Android
versions handle multidex natively, and without any bugs to workaround.

See also b/476359563

Author: ejona86

android-interop-testing/src/androidTest/AndroidManifest.xml

@@ -5,8 +5,7 @@
         android:name="androidx.test.runner.AndroidJUnitRunner"
         android:targetPackage="io.grpc.android.integrationtest" />
 
-    <application
-        android:name="android.support.multidex.MultiDexApplication" >
+    <application>
     </application>
 
 </manifest>

android-interop-testing/src/main/AndroidManifest.xml

@@ -14,8 +14,7 @@
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
-        android:theme="@style/Base.V7.Theme.AppCompat.Light"
-        android:name="androidx.multidex.MultiDexApplication">
+        android:theme="@style/Base.V7.Theme.AppCompat.Light">
         <activity
             android:name=".TesterActivity"
             android:exported="true">