Add WindowMetricsHelper

This metrics now is used for calculating the value reported by
Display#getSize. It could be extended to have more function later.

Also replace getCurrentMetrics usages to make them report
Display#getSize value.

Bug: 148904274
Test: atest WindowMetricsHelperTest

Change-Id: I182b6e63f7c28752eee22839cb9e69f073046c3f