[Boundary interfaces] Add a package file to hide boundary interfaces

Android publishes all public APIs that are not marked with the @hide
annotation.
Boundary interfaces are not supposed to be public to apps. In this CL we
add a file with the sole purpose of hiding the boundary interface
package from apps.

Bug: 788177
Change-Id: I58c9c7a8445c1a3fdfda3164735f5e2c755dfc44
Reviewed-on: https://chromium-review.googlesource.com/918861Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Gustav Sennton <gsennton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536719}

android_webview/support_library/boundary_interfaces/src/org/chromium/support_lib_boundary/package-info.java

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * @hide
+ */
+package org.chromium.support_lib_boundary;