Soong generates kernel_version_for_uffd_gc.txt

In the context of support "--soong-only", kernel_version_for_uffd_gc.txt
needs to be generated by Soong.

Soong will write the BOARD_KERNEL_VERSION to the txt file
in soong-only mode. It will panic if BOARD_KERNEL_VERSION
is not set.

Test: CI
Bug: 383564057
Change-Id: Idb264eaa91424ed9bab5be4988c7eb96334153f5