[Android] Added dummy value for kWidevineKeySystem

Otherwise there is a build error when widevine is disabled.

Chromium change:
https://github.com/chromium/chromium/commit/b27c364310e600c4bb615255fc9a623fdbacf89a

media: Use KeySystemSupport on Clank
Change Clank so that rather than using the CdmMessageFilterAndroid
IPC, it uses KeySystemSupport (a mojo interface). In the browser, any
available CDM is registered with null capabilities, which will be
populated the first time some page attempts to use protected media.
On the renderer side, it will fetch the capabilities when needed.

Tested by playing encrypted content in both regular and incognito mode.
It matches what Chrome currently does.

chrome://media-internals#cdms reports:
Key System: com.widevine.alpha
Robustness: Software Secure
Name:
Version: invalid
Path:
Status: Enabled
Capabilities: {
  "Audio Codecs":["aac","opus"],
  "Encryption Schemes":["CENC","CBCS"],
  "Session Types":["temporary","persistent-license"],
  "Video Codecs":{"h264":[],"hevc":["hevc main"],
                  "vp8":[],"vp9":["vp9 profile0"]}}
Key System: com.widevine.alpha
Robustness: Hardware Secure
Name:
Version: invalid
Path:
Status: Enabled
Capabilities: "No Capability"

Bug: 853336

chromium_src/chrome/renderer/media/chrome_key_systems.cc

@@ -0,0 +1,13 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "third_party/widevine/cdm/buildflags.h"
+
+#if !BUILDFLAG(ENABLE_WIDEVINE)
+// We need this value for DCHECK only.
+inline constexpr char kWidevineKeySystem[] = "com.widevine.alpha";
+#endif  // !BUILDFLAG(ENABLE_WIDEVINE)
+
+#include "src/chrome/renderer/media/chrome_key_systems.cc"

chromium_src/components/cdm/renderer/key_system_support_update.cc

@@ -0,0 +1,13 @@
+/* Copyright (c) 2023 The Brave Authors. All rights reserved.
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#include "third_party/widevine/cdm/buildflags.h"
+
+#if !BUILDFLAG(ENABLE_WIDEVINE)
+// We need this value for DCHECK only.
+inline constexpr char kWidevineKeySystem[] = "com.widevine.alpha";
+#endif  // !BUILDFLAG(ENABLE_WIDEVINE)
+
+#include "src/components/cdm/renderer/key_system_support_update.cc"