Increase Shell APK version to 28.

Increase the shell APKs version, so the existing WebAPKs can be updated to have
the missing android.permission.VIBRATE permission in their AndroidManifest.

Bug: 773663
Change-Id: I92566784492ce37d4ff2102794c3323e6fa834d3
Reviewed-on: https://chromium-review.googlesource.com/716638
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Commit-Queue: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508399}

chrome/android/webapk/shell_apk/shell_apk_version.gni

@@ -12,4 +12,4 @@ template_shell_apk_version = 28
 # if the WebAPK's ShellAPK version is less than |expected_shell_apk_version|.
 # The version should be incremented after a new ShellAPK has been uploaded to
 # the WebAPK Minting Server.
-expected_shell_apk_version = 26
+expected_shell_apk_version = 28