Do not use stableDisplaySize for scale factor

The stableDisplaySize doesn't account for different
displays, and only returns the stable display size of one of the
displays. Instead of using stableDisplaySize, use the
maxResolutionDisplayMode for the current display to determine
whether the resolution of the device changed.

Test: manually check that lock icon is an expected size
on a device with multiple displays
Test: UDFPS is the correct size at different display resolutions
Fixes: 246631157

Change-Id: Iea933733f00ac7b8446ce6fc5825475e4b490655