Add theme mode design let user can choose theme self

Add a ContentObserver on WallpaperManagerService to
listen THEME_MODE value change. When changed, call
notifyWallpaperColorsChanged and set WallpaperColors
color hint by the current mode.

Bug: 63903361
Test: manual
Change-Id: I4f7aa7b184565b1bb23c7f4f1f07fc310dac7546
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index 5b7fc6e..932ffec 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -591,6 +591,7 @@
                  Settings.Secure.SHOW_ROTATION_SUGGESTIONS,
                  Settings.Secure.SKIP_FIRST_USE_HINTS, // candidate?
                  Settings.Secure.SMS_DEFAULT_APPLICATION,
+                 Settings.Secure.THEME_MODE,
                  Settings.Secure.TRUST_AGENTS_INITIALIZED,
                  Settings.Secure.TV_INPUT_CUSTOM_LABELS,
                  Settings.Secure.TV_INPUT_HIDDEN_INPUTS,