AW: add build.gradle to build boundary interfaces in AndroidX

Add build.gradle and AndroidManifest.xml files that are needed to build
these interfaces as an independent target in AndroidX repo.

These files are used by AndroidX build scripts only and should not be
used inside chromium project.

Bug: https://issuetracker.google.com/issues/140178227
Change-Id: I7686c7e49bfc023dd818210e583b7f535fe9b61c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823119
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700292}

android_webview/support_library/boundary_interfaces/AndroidManifest.xml

+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="org.chromium.support_lib_boundary">
+  <!-- This AndroidManifest file only exists to provide package name needed by
+       gradle script to build these interfaces as part of AndroidX. -->
+</manifest>

android_webview/support_library/boundary_interfaces/build.gradle

+// Copyright 2019 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.
+
+// This file is to build WebView boundary interfaces as part of the AndroidX webkit library.
+// It is not meant to be used or build any targets in chromium project.
+
+import androidx.build.SupportConfig
+
+plugins {
+    id('com.android.library')
+}
+
+android {
+    // COMPILE_SDK_VERSION provided by AndroidX build scripts to build all AndroidX modules.
+    compileSdkVersion SupportConfig.COMPILE_SDK_VERSION
+
+    sourceSets {
+        main.manifest.srcFile 'AndroidManifest.xml'
+        main.java.srcDirs += ['src']
+    }
+
+    buildTypes.all {
+        consumerProguardFiles 'proguard.flags'
+    }
+}