Fix ActivityManagerVrDisplayTests flacky

DisplayPolicy would decide if the external display should
support navigation bar by checking supportsSystemDecorations.
For VR virtual display, it will be used to run and render
2D app within a VR experience, and it shouldn't support
the system decoration.

Bug: 128313423
Test: atest ActivityManagerVrDisplayTests
Test: atest ActivityManagerMultiDisplayTests
Change-Id: I2594ac88877e157fe091c0c98b1098885d29bde6