Enable android.net to reuse OkHttpClient creation logic

Pulling out static factory methods from HttpHandler and
HttpsHandler. android.net will call these to create
OkHttpClient instances which they can then further tweak
for their own purposes.

Bug: 17420465

(cherry-picked from commit c19427fc)

Change-Id: Id4825f6b9599c79594f977fd726beb3465b96e19