Check isHomeSupported() before notifying wallpaper

instead of isSystemDecorationsSupported() when display adds system decorations, because wallpaper is also supposed to be shown when isHomeSupported of DisplayWindowSettings is enabled. See Java doc of VirtualDisplayConfig#setHomeSupported().

This fixes virtualDeviceHome_noHomeComponent_showsWallpaper() and virtualDeviceHome_withHomeComponent_showsWallpaper() in VirtualDeviceHomeTest.

Bug:398397359
Change-Id: I1da49b4d522ae333131939a340e8b69cb544b505
Test: atest VirtualDeviceHomeTest
Flag:com.android.server.display.feature.flags.enable_display_content_mode_management
et