libhwui: Use Vulkan by default

Pixel has been using Vulkan in libhwui for several years now. We
strongly recommended using it in the Android 14 (U) CDD, and we plan to
make it MUST in Android 15 (V). Update the default to match.

Bug: 293371537
Bug: 301558076
Test: CtsUiRenderingTestCases, CtsGraphicsTestCases
Change-Id: I106381fadf6a686127cb6120afbaef53db5c4aa8