HdmiManager: Rework getRequestedResolution()

* Using WindowManager presents a strange issue, where the sysfs node
    that we get the current resolution from is not available.
* If not available, fall back to ubootenv.var.outputmode value
* If this prop comes back null or the resolution is not available,
    fall back to 720p

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>