[omaha-client] common::App::new and related cleanups

Some suggested cleanups in earlier code that was caught in a more recent
code review.

A) This adds an App::new<I, V>(I, V) -> Self constructor and
a similar one called ::with_fingerprint().  These use type parameters
and Into<String> and Into<Version>, and as a result, allow for much
tidier code when constructing an App.

B) This switches a list traversal to a functional .find() call in the
RequestBuilder.  Which again, tidies things up.

Bug: PKG-679
Test: fx run-test omaha_client_lib_tests

Change-Id: If0b595355b2578be43e12722b1b9307496db91fa