Run @FlakyTest tests in staging.

This will give us a way to keep an eye on how flaky they are without
needing to remove @FlakyTest every time we want to check.

Bug: 177444865
Test: atest --test-mapping frameworks/base/packages/SystemUI:staged-platinum-postsubmit
Change-Id: Ia6bf62e15555467727c2b108e508bec68be5b0cd