util-app: Update visibility for Flag constructors
util-app: Update visibility for Flag constructors

Problem

Useful constructors for `c.t.app.Flag` are private to `c.t.app`.

Solution

Update the visibility to be private to `c.twitter`.

Result

These constructors can be called by other code in the `c.twitter`
namespace.

RB_ID=908607

BuoyantIO/twitter-util,BuoyantIO/twitter-util,twitter/util,BuoyantIO/twitter-util,twitter/util,twitter/util,BuoyantIO/twitter-util