This brings OkHttp and okio exactly in line with upstream commits
with no local changes.

Corresponding upstream commits:
okhttp:6e236ce3b80f21369dc544f0e1053ff71be8689b (= parent-2.7.5)
okio: 02481cc0cc84bc92e3eab6d5212a226496f56a7e

The okio commit differs from the one in the previous pull from
Sep 2015 (AOSP commit 71b9f47b)
only by a single upstream commit, the switch to 8 KiB segments.
That commit was previously cherry-picked in AOSP. This CL will
temporarily revert the AOSP changes to okio, but those
AOSP changes to okio will be reapplied in the subsequent CL.

Compilation and tests do not pass after this CL, they will only
pass at the end of the chain of 11 CLs going in at the same time.
9 of these 11 CLs are in external/okhttp, the others affect
libcore and frameworks/base.

Details of behavioural changes introduced by this upgrade are at:
https://docs.google.com/document/d/19PF3Exd_q32gAGCiRFWRf0Pq_xrIWs-cRViHkFTxJg8/edit

This CL includes files that are not used in Android, such as
 - top level dot files (.travis.yml etc.)
 - subdirectories okurl, okhttp-apache, samples, which aren't used
 - tests in okhttp-hpacktests, okhttp-ws-tests that aren't run
   or test functionality that we aren't used

Test: I've run the following tests *at the end* of the chain of
commits, in cts-tradefed:
1.) run cts -p android.core.tests.libcore.package.harmony_java_net
2.) run cts -c libcore.java.net.URLConnectionTest
3.) run cts -p android.core.tests.libcore.package.okhttp
4.) run cts -p android.core.tests.libcore.package.libcore

1.-3.) all passed
4.) had 24 unrelated failures per b/29496407 and b/29744850

Change-Id: Id798d6cf49fa4a7a4ab8ae3b699a38104bf42db3