Fix Android build/test (#1706)

ADO was upgrading our Gradle version underneath us, so this pins us to a particular gradle version that works with our code as is.

I did explore refactoring our code to work with the newer Gradle versions, but encountered some issues. We can go deeper on this later if needed.

Also removes some unused Android app code in our test folder (all we need is the android test code)

Author: timtay-microsoft

iot-e2e-tests/android/app/src/main/AndroidManifest.xml

@@ -3,20 +3,4 @@
           package="com.microsoft.azure.sdk.iot.android">
 
     <uses-permission android:name="android.permission.INTERNET"/>
-
-    <application
-        android:allowBackup="true"
-        android:icon="@mipmap/ic_launcher"
-        android:label="@string/app_name"
-        android:supportsRtl="true"
-        android:theme="@style/AppTheme">
-        <activity android:name="com.microsoft.azure.sdk.iot.android.MainActivity">
-            <intent-filter>
-                <action android:name="android.intent.action.MAIN"/>
-
-                <category android:name="android.intent.category.LAUNCHER"/>
-            </intent-filter>
-        </activity>
-    </application>
-
 </manifest>