DisplayContent: Remove unnecessary setRelativeLayer

This code is doing setRelativeLayer 1, relative to its own
parent. This is equivalent to setLayer 1, but much more expensive
in SurfaceFlinger. Fix it up as part of the dream of
deprecating/removing setRelativeLayer.

Bug: 214315611
Test: Existing tests pass
Change-Id: I0c83180e5c56488a0dd8b97083578457f4be5256