Update document of WindowMetrics#getSize

Update document to notify that reported size is different between
WindowMetrics#getSize and Display#getSize and provide a sample
code to transform between usages.

Test: build & run
Bug: 128338354
Change-Id: I06aee9aaf903c3864fbb0fe89ea1d3e15506fe1c
(cherry picked from commit 2de0f42d)