Add persistent=true to the Android manifest

Includes persistent=true to the manifest to allow for a
lower oom_adj score when running the browser instance
on Android Things so low memory killer doesn't
incorrectly tear down required processes when under
memory pressure.

Bug: internal
Change-Id: I34e04f8d4485c613fef3187a6e305e17b9e51855
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1653652
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Auto-Submit: Brett Kilty <brettk@google.com>
Cr-Commit-Position: refs/heads/master@{#668444}

chromecast/browser/android/BUILD.gn

@@ -13,6 +13,7 @@ cast_shell_android_manifest =
 
 manifest_variables = [
   "cast_build_enable_background_activities=$enable_background_activities",
+  "is_android_things=$is_android_things",
   "is_android_things_non_public=$is_android_things_non_public",
 ]
 

chromecast/browser/android/apk/AndroidManifest.xml.jinja2

@@ -27,6 +27,9 @@
                   android:exported="true"
                   android:hardwareAccelerated="true"
                   android:launchMode="standard"
+                  {% if is_android_things == "true" %}
+                  android:persistent="true"
+                  {% endif %}
                   android:screenOrientation="landscape"
                   android:taskAffinity=".CastWebContentsActivity"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"