Revert "Remove android_manifest_for_lint from BUILD.gn files"

This reverts commit 340106ad4659faca563509016097a091bfb5289f.

Reason for revert: Broke internal project.

Original change's description:
> Remove android_manifest_for_lint from BUILD.gn files
> 
> The Chromium Roll into WebRTC isn't flowing
> The first CL that caused the problem is https://webrtc-review.googlesource.com/c/src/+/175140/
> The error is: ERROR at //examples/BUILD.gn:104:33: Assignment had no effect. android_manifest_for_lint = "androidapp/AndroidManifest.xml"
> android_manifest_for_lint has ben removed so update BUILD files that use that feature to reflect this.
> 
> BUG=None
> 
> Change-Id: Ic3eb16eab8e4a4ab87daac9998d7a07373fec493
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175569
> Commit-Queue: Courtney Edwards <courtneyfe@google.com>
> Reviewed-by: Artem Titov <titovartem@webrtc.org>
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31299}

TBR=mbonadei@webrtc.org,titovartem@webrtc.org,courtneyfe@google.com,courtneyfe@chromium.org

Change-Id: I6055b4363254d353d116805aac4ec63d7c5c7c59
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: None
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175622
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31303}

examples/BUILD.gn

@@ -101,6 +101,7 @@ if (is_android) {
 
   rtc_android_library("AppRTCMobile_javalib") {
     testonly = true
+    android_manifest_for_lint = "androidapp/AndroidManifest.xml"
 
     sources = [
       "androidapp/src/org/appspot/apprtc/AppRTCAudioManager.java",

rtc_tools/network_tester/BUILD.gn

@@ -115,6 +115,7 @@ if (is_android) {
 
   rtc_android_library("NetworkTesterMobile_javalib") {
     testonly = true
+    android_manifest_for_lint = "androidapp/AndroidManifest.xml"
 
     sources = [
       "androidapp/src/com/google/media/networktester/MainActivity.java",