DM: Cleanup SF APIs for creating / destroying virtual display

Follow-up CL for cleanup requested by SF owner in http://ag/27202517.

Bug: 339525838
Bug: 137375833
Bug: 194863377
Test: atest libsurfaceflinger_unittest
Test: atest DisplayServiceTests
Flag: EXEMPT refactor
Change-Id: Iabe65dd64c0a1f325e215a9ace0bf672a423b589