Apply upstream commit: Allow HTAB in header values.

This applies upstream commit
02b08fbde7b1726d7a4c0dc971152751ac82ca0a

Due to deviation between upstream and AOSP, the change was applied
manually.

RFC 7230 section 3.2 allows HTAB ('\t', '\u0009') inside header
values as long as there is not more than one in a row:
  https://tools.ietf.org/html/rfc7230#section-3.2

Before this CL, OkHttp previously disallowed HTAB in header values.
This CL changes behavior to allow any number of consecutive HTABs
inside a header value; this is more permissive than the RFC, but
is consistent with how OkHttp currently treats space characters
(' ', '\u0020').

Bug: 30799514

Test: run cts -m CtsLibcoreTestCases
Test: run cts -m CtsLibcoreOkHttpTestCases

Change-Id: I0dd68d1697affaf739167174970d52e466a2bc16
(cherry picked from commit 1917ec96)