Android - Adding at least one Activity with an ACTION-VIEW intent-filter after Updating SDK version 23

From official documentation :

To enable Google to crawl your app content and allow users to enter your app from search results, you must add intent filters for the relevant activities in your app manifest. These intent filters allow deep linking to the content in any of your activities. For example, the user might click on a deep link to view a page within a shopping app that describes a product offering that the user is searching for.

Using this link Enabling Deep Links for App Content you'll see how to use it.

And using this Test Your App Indexing Implementation how to test it.

The following XML snippet shows how you might specify an intent filter in your manifest for deep linking.

<activity
    android:name="com.example.android.GizmosActivity"
    android:label="@string/title_gizmos" >
    <intent-filter android:label="@string/filter_title_viewgizmos">
        <action android:name="android.intent.action.VIEW" />
        <category android:name="android.intent.category.DEFAULT" />
        <category android:name="android.intent.category.BROWSABLE" />
        <!-- Accepts URIs that begin with "http://www.example.com/gizmos” -->
        <data android:scheme="http"
              android:host="www.example.com"
              android:pathPrefix="/gizmos" />
        <!-- note that the leading "/" is required for pathPrefix-->
        <!-- Accepts URIs that begin with "example://gizmos” -->
        <data android:scheme="example"
              android:host="gizmos" />

    </intent-filter>
</activity>

To test via Android Debug Bridge

$ adb shell am start
        -W -a android.intent.action.VIEW
        -d <URI> <PACKAGE>

$ adb shell am start
        -W -a android.intent.action.VIEW
        -d "example://gizmos" com.example.android

You can remove the warning by adding the below code in <intent-filter> inside <activity>

<action android:name="android.intent.action.VIEW" />

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.example.app"
tools:ignore="GoogleAppIndexingWarning">

You can remove the warning by adding xmlns:tools="http://schemas.android.com/tools" and tools:ignore="GoogleAppIndexingWarning" to the <manifest> tag.