Publish SystemWebViewShell.apk

We would like to publish system webview shell automatically such that
WPT test can use it. Note that this is CI bot and we do not build per
commit.

Also, system webview shell is a very useful utility by itself.

Once this lands, it will be packed into chrome-android.zip in one of the
subdirectories here:

https://commondatastorage.googleapis.com/chromium-browser-snapshots/
index.html?prefix=Android/

The APK size is 1.5 MB. The current chrome-android.zip is about 80 MB,
so this will hopefully not increase the zip file too much.

Bug: 993112
Change-Id: If35038968bdb52b2ed23086b0d9d7b75fa3c8906
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761234Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688340}

chrome/tools/build/android/FILES.cfg

@@ -35,4 +35,8 @@ FILES = [
     'filename': 'apks/ChromePublic.apk',
     'buildtype': ['dev', 'official'],
   },
+  {
+    'filename': 'apks/SystemWebViewShell.apk',
+    'buildtype': ['dev', 'official'],
+  },
 ]