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 c19427fc6c8c1be67970437643c8fe90e328fc6c)

Change-Id: Id4825f6b9599c79594f977fd726beb3465b96e19
Merge "Enable android.net to reuse OkHttpClient creation logic"