[Chromecast] Generate random SN when real SN is "Unknown".

The generated serial is stored in SharedPreferences in order to keep it
consistent between reboots.

Bug: internal b/111077464
Test: look at logs
Change-Id: If988936bedd4fe9a9946ef2671a747dfc622ecf7
Reviewed-on: https://chromium-review.googlesource.com/1237524
Commit-Queue: Thoren Paulson <thoren@chromium.org>
Reviewed-by: Simeon Anfinrud <sanfin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594392}

chromecast/browser/android/BUILD.gn

@@ -108,6 +108,7 @@ android_library("cast_shell_java") {
     "$java_src_dir/org/chromium/chromecast/shell/CastCrashUploader.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastCrashUploaderFactory.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastMetricsHelper.java",
+    "$java_src_dir/org/chromium/chromecast/shell/CastSerialGenerator.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastSysInfoAndroid.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastWebContentsActivity.java",
     "$java_src_dir/org/chromium/chromecast/shell/CastWebContentsComponent.java",

chromecast/browser/android/apk/src/org/chromium/chromecast/shell/CastSerialGenerator.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.
+
+package org.chromium.chromecast.shell;
+
+import android.content.SharedPreferences;
+import android.util.Base64;
+
+import org.chromium.base.ContextUtils;
+
+import java.security.SecureRandom;
+
+/**
+ * Helper for generating a serial number on Android.
+ */
+final class CastSerialGenerator {
+    private static final String GENERATED_SERIAL_KEY = "generated_serial";
+    // 20 characters in Base64, the most supported by DCS.
+    private static final int GENEREATED_SERIAL_BYTES = 15;
+
+    private static String getRandomSerial() {
+        byte[] bytes = new byte[GENEREATED_SERIAL_BYTES];
+        new SecureRandom().nextBytes(bytes);
+        return Base64.encodeToString(bytes, Base64.URL_SAFE | Base64.NO_PADDING | Base64.NO_WRAP);
+    }
+
+    private static String generateSerial() {
+        SharedPreferences prefs = ContextUtils.getAppSharedPreferences();
+        String result = prefs.getString(GENERATED_SERIAL_KEY, null);
+        if (result != null) return result;
+        result = getRandomSerial();
+        prefs.edit().putString(GENERATED_SERIAL_KEY, result).apply();
+        return result;
+    }
+
+    private static final String GENERATED_SERIAL = generateSerial();
+
+    public static String getGeneratedSerial() {
+        return GENERATED_SERIAL;
+    }
+}
\ No newline at end of file

chromecast/browser/android/apk/src/org/chromium/chromecast/shell/CastSysInfoAndroid.java

@@ -20,7 +20,8 @@ public final class CastSysInfoAndroid {
     @SuppressLint("HardwareIds")
     @CalledByNative
     private static String getSerialNumber() {
-        return Build.SERIAL;
+        if (!Build.SERIAL.equals(Build.UNKNOWN)) return Build.SERIAL;
+        return CastSerialGenerator.getGeneratedSerial();
     }
 
     @SuppressLint("HardwareIds")