Opt hwui_unit_tests (and benchmarks) out of test runner sharding

Testing this to see if it helps weird issues seen on b/263382503. This
has been suggested to be a culprit for other very similar issues in the
past.

Docs: https://source.android.com/docs/core/tests/tradefed/testing/through-suite/android-test-tuning
Bug: 263382503
Test: presubmits
Change-Id: Ic2729d41019ad9910ad57a984d63903c1c97fd11