Split ZenMode.getIcon() into getIconKey() and ZenIconLoader.getIcon()

Also return a ZenIcon.Key together with the drawable, indicating where the resource was loaded from (this will be needed for status bar).

Bug: 360399800
Bug: 361597532
Test: atest ZenModeTest ZenIconLoaderTest
Flag: android.app.modes_ui
Change-Id: Ic2f3191b24373ce7ec5e90961fed5af2f15b49be
12 files changed