Add a CTS test for the header map populated by CacheManager.getCacheFile()

We only test the case where the header map is not populated, as we don't place
hard guarantees on exactly how and when it is populated.

See https://android-git.corp.google.com/g/#/c/166978

Change-Id: Ib14030c78962ad91776ad152f7955f8f0a8ce4f9
1 file changed