Android modifications to OkHttp

Re-introduce Android integration classes and
make the modifications necessary to get OkHttp/Okio
to compile against Android.

The only Android-specific change made over prior versions
of this code is in Platform to remove the toURILenient()
method (no longer used by OkHttp).

This commit also includes pending PR
https://github.com/square/okhttp/pull/1858
to fix two tests on Android:
com.squareup.okhttp.URLConnectionTest#connectViaHttpsWithSSLFallback
com.squareup.okhttp.CallTest#recoverFromTlsHandshakeFailure_Async

Bug: 22943975
Bug: 23553256
Change-Id: I4c718c96fbd5fe3f2353ce422f8de3b95d0b75ee