okhttp: Convert to internal ConnectionSpec eagerly

[ejona86]

This allows ProGuard to remove OkHttp's ConnectionSpec in most cases,
saving about 40 methods. The savings won't be realized until
DEFAULT_CONNECTION_SPEC is removed.

[ejona86]

@ericgribkoff, so the DEFAULT_CONNECTION_SPEC is public. That makes it a bit unclear on what to do. It may be best to just give up on this particular detail.

[ericgribkoff]

Not sure what to do here, unless we can deprecate DEFAULT_CONNECTION_SPEC and eventually delete it (internally, it seems no one is relying on this). Otherwise the best I can see is having a parallel INTERNAL_DEFAULT_CONNECTION_SPEC, with no internal references to OkHttp's DEFAULT_CONNECTION_SPEC...this saves exactly 4 methods after proguard, so would not be worth the change.

[ejona86]

@ericgribkoff, made the change as we discussed. PTAL