Allow changing composition from DISPLAY_DECORATION to DEVICE

After some discussion, we've decided to always set the ScreenDecorations
to DISPLAY_DECORATION. HWC can decide, based on the format, whether to
truly treat it as DISPLAY_DECORATION, or to change to DEVICE
composition.

Previously we were concerned that SurfaceFlinger couldn't use this
information (if we provided it). Looking to the future, it's possible SF
may use this information to help make a better plan for the next frame.

Bug: 193170859
Test: manual
Change-Id: I1da3199bacf5877e88876249c60c0a33fcd44c1e