Exclude kernel_headers from vendor snapshot

Vendor snapshot is supposed to have AOSP-defined modules which are
installed to vendor. kernel_headers are excluded from snapshot because
they always depend on vendor, regardless of locations they are defined.

Bug: 157106227
Test: m vendor-snapshot
Change-Id: If47634678797973266fcf502739216daf28ad790
Merged-In: If47634678797973266fcf502739216daf28ad790
(cherry picked from commit 65ca36a7)