Adding test-mapping group for large screen specific tests

"large-screen-postsubmit" filters test by @LargeScreenOnly annotation.
See sample test in ag/15795721.

Bug: 191260984
Test: atest --test-mapping frameworks/base/packages/SystemUI:large-screen-postsubmit
Change-Id: Iad98e54521ad0ee782b5e28a3cb8c885192a6ec5
Merged-In: Iad98e54521ad0ee782b5e28a3cb8c885192a6ec5