Speed up vendor snapshot header globs

The vendor snapshot header globs were repeated 9 times, once for each
possible header suffix.  Globbing is not a fast operation, so this
took 9.68s of CPU time in my AOSP builds.  Replace it instead with a
single glob, followed by filtering down to the desired header
extensions.

Test: m checkbuild
Change-Id: I9c8f8cdad8cbffc0a42217659a48dd9d676e4e4c