cherrypick from mnc-docs docs: Updated APK Signature Scheme v2 doc.

<h2 id="apk_signature_v2">APK signature scheme v2</h2>

<p>The PackageManager class now supports verifying apps using the APK

signature scheme v2. The APK signature scheme v2 is a whole-file signature scheme

that significantly improves verification speed and strengthens integrity

  guarantees by detecting any unauthorized changes to APK files.</p>

<p>To maintain backward-compatibility, an APK must be signed with the v1 signature

scheme (JAR signature scheme) before being signed with the v2 signature scheme.

With the v2 signature scheme, verification fails if you sign the APK with an

  additional certificate after signing with the v2 scheme. </p>

<p>APK signature scheme v2 support will be available later in the N Developer

Preview.</p>

<p>

  Android N introduces APK Signature Scheme v2, a new app-signing scheme that

  offers faster app install times and better protection against unauthorized

  alterations to APK files. Android Studio 2.2 and Gradle provide built-in

  support for APK Signature Scheme v2.

</p>

<p>

  Although we recommend applying APK Signature Scheme v2 to your app, the new

  scheme is not mandatory. If your app doesn't build properly when using the

  APK Signature Scheme v2, you can use the traditional signing scheme—which

  uses JAR signing—instead. To use the traditional scheme, open the

  module-level <code>build.gradle</code> file and add the

  <code>v2SigningEnabled</code> parameter to your release signing

  configuration, setting this parameter's value to <code>false</code>:

</p>

<pre>

  android {

    ...

    defaultConfig { ... }

    signingConfigs {

      release {

        storeFile file("myreleasekey.keystore")

        storePassword "password"

        keyAlias "MyReleaseKey"

        keyPassword "password"

        <strong>v2SigningEnabled false</strong>

      }

    }

  }

</pre>

<p>

  For more information, see the following guides, which describe how to <a href=

  "{@docRoot}studio/tools/publishing/app-signing.html#release-mode"> sign an app

  in Android Studio</a> and how to <a href=

  "{@docRoot}studio/tools/building/configuring-gradle.html#signing"> configure

  the Gradle build file for signing apps</a>.

</p>

<h2 id="scoped_directory_access">Scoped directory access</h2>