SkiaVkRenderEngine: Use HIGH priority if REALTIME is not available

Not all drivers that support the "VK_EXT_global_priority" extension
support "VK_QUEUE_GLOBAL_PRIORITY_REALTIME_EXT".  Instead of assuming
REALTIME by the existence of the extension, properly query for the
highest priority, using HIGH if available.

Bug: 281820575
Test: Manual testing
Change-Id: I846e829b775c7df90a44a0c97c0007cc6b44b438