Convert DisplayList to a value-type wrapper

Make DisplayList its own type instead of an alias,
pushing the Skia aspect behind it mostly. Removes a bunch
of manual memory management and opens the door to DisplayList
being a union type with multiple implementations

Test: builds (somehow), boots, hwuiunit passes, CtsUiRendering passes
Change-Id: I1d7806aa3afc5d9ece08b06959920078a5814c59