Rename DisplayConfig to DisplayMode

This CL continues the raneming of display "configs" to
display "modes". The goal of this is to have the same
names as in the java code and the public display APIs.

Additionally in this CL we move DisplayConfig (from libui)
to the namespace android::ui. This is to prevent conflict
with the SurfaceFlinger's internal android::DisplayMode.
This is consistent with the neighboring classes which
are also in the ui namespace.

Also the type of the parameter defaultMode of
{s,g}etDesiredDisplayModeSpecs is changed to size_t
for consistency with the rest of the code. Appropriate
error handling is added for this.

Bug: 159590486
Bug: 179158858
Test: presubmit
Change-Id: I31e5be1f2223a9ec9340789ce3dc5738eceaf40f