Remove minLayer and maxLayer from display screenshot.

When taking a full display screenshot, there's no longer a need for min
and max layer. Instead, the entire display will be screenshotted. If
only some layers should be screenshotted, then use the captureLayers method
instead.

Also, condensed the methods so there is only one screenshot method for
displays that returns a GraphicBuffer. SurfaceControl can then convert
it to the desired format, like Bitmap or Surface.

Test: Rotation, adb shell screencap, power + volume down, bugreport.
Change-Id: Ia5a293e89d2cf3811fd06ffcbafeee389a32840e