Suppress MissingPrefix using tools attribute

Suppressing lint errors using comments is an undocumented feature of the
linter, and suppressing using the tools:ignore attribute should be
preferred.

Suppressing using comments becomes a problem when using the manifest
merger introduced in
https://chromium.googlesource.com/chromium/src/build/+/6ada47bc7950dd870a3a82458790a0cc0b7ba649
as it reformats the comments slightly:
  <!--suppress MissingPrefix -->
becomes
  <!-- supress MissingPrefix -->
which causes the linter to disregard the suppression.

Bug: 740657
Change-Id: I8e365744d089271c390254e7c958b24b81043766
Reviewed-on: https://chromium-review.googlesource.com/566860
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Ingemar Ådahl <ingemara@opera.com>
Cr-Commit-Position: refs/heads/master@{#18971}
diff --git a/webrtc/examples/androidtests/AndroidManifest.xml b/webrtc/examples/androidtests/AndroidManifest.xml
index 1fd96dd..7ce02dc 100644
--- a/webrtc/examples/androidtests/AndroidManifest.xml
+++ b/webrtc/examples/androidtests/AndroidManifest.xml
@@ -10,6 +10,7 @@
 
 <manifest
       xmlns:android="http://schemas.android.com/apk/res/android"
+      xmlns:tools="http://schemas.android.com/tools"
       package="org.appspot.apprtc.test">
 
     <uses-permission android:name="android.permission.RUN_INSTRUMENTATION" />
@@ -19,10 +20,10 @@
         <uses-library android:name="android.test.runner" />
     </application>
 
-    <!-- Needed for chromium-junit4 tag. crbug.com/640116
+    <!-- tools:ignore needed for chromium-junit4 tag. crbug.com/640116
          TODO(sakal): Remove once the tag is no longer needed. -->
-    <!--suppress MissingPrefix -->
     <instrumentation android:name="android.support.test.runner.AndroidJUnitRunner"
+        tools:ignore="MissingPrefix"
         android:targetPackage="org.appspot.apprtc"
         android:label="Tests for AppRTCMobile"
         chromium-junit4="true"/>