Kernel headers intermediary repo

Selects qti_kernel_headers repo based on kernel version so that multiple versions can live inside on AOSP tree.

Needs following changes:

SOONG_CONFIG_NAMESPACES += qti_kernel_headers
SOONG_CONFIG_qti_kernel_headers := version
SOONG_CONFIG_qti_kernel_headers_version := 4.14

common-headers needs to rename the module name from qti_kernel_headers to qti_kernel_headers_4.14 (based on version).