Allow ProxyBuilder to call hidden APIs

[drewhannay]

Consider the following scenario:

• A test creates a mock of class A, which contains a blacklisted API
• ProxyBuilder cannot see the blacklisted method, so it does not create a proxy for that method (calling getDeclaredMethods() on the class omits the blacklisted methods).
• The test now passes the mock of class A to another system class, B.
• B contains code that calls the blacklisted method, and consequently calls through to the real implementation of that blacklisted method.
• The real implementation of the blacklisted method accesses some field which has not been initialized and therefore crashes.

Whitelisting ProxyBuilder to be able to call hidden APIs will allow it to properly create proxies for those methods and prevent this type of crash.

[drewhannay]

@moltmann I'd love to get your review on this to double check that this is the right solution.

[moltmann]

Sounds reasonable