DO NOT MERGE Relax N CTS about whether \t is allowed in header values

Android N's Http(s)URLConnection initially forbade '\t' occurring in header
values when it should be allowed; this was fixed by [1] which will be
part of a future Android release. It's permitted for vendors shipping
Android N to apply this fix. Therefore, this CL relaxes CTS for Android N
to allow either behavior. CTS for APIs > 23 will enforce that \t is
allowed.

[1] AOSP commit dcabb633

Test: vogar libcore.java.net.URLConnectionTest#testRequestHeaderValidation
      against (a) N (b) a recent build with http://r.android.com/272102
      During local testing, temporarily added log statements after the two
      asserts to verify that the correct branch was taken on each device.

Bug: 30799514
Change-Id: I4f37246337045574f5ab498d5e98b9c98e6feb74