FP3: surfaceflinger: Disable HAL virtual displays

FP3's legacy HAL currently doesn't support this properly. In test cases,
the first frame rendered to the virtual display is always just empty.

With HAL virtual displays being disabled, SurfaceFlinger will fall back
to GPU virtual displays, which seems to work well enough.

Issue: FP3-A12#13
Test: run cts -m CtsViewTestCases -t android.view.cts.ASurfaceControlBackPressureTest#testSurfaceTransaction_setEnableBackPressure
Test: run cts -m CtsViewTestCases -t android.view.cts.SurfaceViewSyncTest#testEmptySurfaceView
Change-Id: I9adfd5bc6aca2046b5541dccf5c327c41e5bb016
(cherry picked from commit f56669b833bd27cb1aee8080afef60fbc5d28ac9)