libui: use __ANDROID_API_V__

__builtin_available() now works with preview SDK version (V in this
case). No need to use __ANDROID_API_FUTURE__ which was a workaround.

__builtin_available(andorid 35, *) evaluates to true if
- the platform SDK version is 35 or higher
- or, the platform is a preview (not REL)

Bug: 322384429
Test: CF loads mapper from vapex
Change-Id: I264cf444a0fb163e445471920930b6be3930dd03