kernel_headers: Fix headers not detected in incremental builds

Currently the Soong logic doesn't kick in if a change updates
headers but doesn't update gen_headers_arm*.bp files for
incremental builds. Fix the issue by making inputs to the
kernel_headers.py script as variables instead of a list.

Change-Id: I9737f6d655848458bfda70b8c6f4d9aa896a98fc
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>