[dev][display] Add Provider interface

The new Provider interface replaces both openat and the get_handle
ioctl. The new approach to getting a controller client is to:
1. Open the device-controller device. This no longer creates a new
instance.
2. Call one of the new Provider methods on the service handle to the
device.
3. Maintain the connection by keeping alive two channels: one for the
device instance, and one for the FIDL interface.

Eventually those two channels should be merged, but that's blocked on
support for llcpp, see ZX-3889.

ZX-2826 #done
ZX-3277 #comment removes open_at from the display driver

Tests:
Boot device (tests virtcon)
display_test
gfxtest (tests framebuffer library)
vkcube_on_fb (tests vulkan swapchain library)
scenic app (tests DisplayWatcher)
recovery app (tests rust framebuffer library)
gfxlatency

Can't run:
display_capture_test (no HW)

Change-Id: Ie187e43692ed7c947920511188c98da38c9843f3