[w-goog]

It's happening!!!!!!!! First of a stack of ~9 PRs removing OCMock & GULSwizzler from the unit tests. This one takes out the NSBundle swizzle in GIDFakeMainBundle and the mocked NSBundle / NSUserDefaults in the migration test.

• New GIDBundle protocol covering the two things the SDK reads from a bundle (bundleIdentifier and objectForInfoDictionaryKey:), with NSBundle conforming and a GIDFakeBundle for tests.
• GIDSignIn, GIDAuthStateMigration, and GIDSignInCallbackSchemes take the bundle (and the first two, user defaults) through new private designated initializers. The existing initializers pass mainBundle / standardUserDefaults.
• GIDFakeMainBundle is now a GIDFakeBundle subclass with the same helpers - no more swizzling!!! :D
• The migration tests now use the fakes, or real UIKit constructs where applicable.

Note for reviewers: the OS routes callbacks by the main bundle regardless of what's injected, and the URL scheme check + legacy keychain fingerprint both read the injected one - so, the bundle has to be the running app's. I documented that on the initializers (rather than enforcing it), as they're private.

Known follow-up: GIDAppCheck still reads GIDClientID from the main bundle on its own...on my TODO list as future work.