vulkan: Conditionally enumerate KHR_shared_presentable_image

This requires:
- GPDP2 present and enabled
- ANDROID_native_buffer V7 implemented
- Driver supporting at least one shared presentation mode

This unpicks the weird case of the hal having to expose the
extension while the api surface was all implemented in the
loader.

V3: Single flag

Change-Id: I336af1a9d2549b74631c7a93a2c3e0e1469a9622
Test: build