Refactor onDisplayReady to onDisplayAddSystemDecorations

Renames onDisplayReady() to onDisplayAddSystemDecorations() to maintain
consistency with onDisplayRemoveSystemDecorations().

Previously, onDisplayReady() was sent regardless of the return value of
isSystemDecorationsSupported(). In this CL,
onDisplayAddSystemDeocrations() is only sent when
isSystemDecorationsSupported() returns true.

Bug: 390591772
Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: atest CommandQueueTest WallpaperManagerServiceTests
Test: adb shell settings put secure mirror_built_in_display {1|0}
Change-Id: I5d4289ac303a79948c9ac51da04e8b486f4e2cf6