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Add basic library support for grpc Metadata through GrpcHandler (#505)

The primary goal of this PR is to support the usage of the class `io.grpc.Metadata` in a way that feels natural to how NullAway already handles maps. 

Handling `io.grpc.Metadata` is complicated by two factors:

* It implements `containsKey(...)` and `get(...)` methods that appear similar to the API of java maps, without actually implementing `java.util.Map`. Thus our existing support for maps can't handle this class, and this likely breaks developer expectations unless they are familiar with the implementation of both the grpc library and NullAway.
* Keys for `io.grpc.Metadata` aren't ever primitive types, but instead are usually references to fields of type `io.grpc.Metadata.Key<T>`. These fields are usually static and final (and the type is immutable), so they can safely be treated as constants, but our existing handling of compile-time constants in access paths is not enough here.

To solve this, we:

1) Implement equivalent logic to our usual map handling as a handler (`GrpcHandler`), which sets `x.y.z.get($foo)` as `@NonNull` on the then-branch whenever it sees a method node representing `x.y.z.containsKey($foo)`.
2) Improve on our handling of AccessPaths which include methods whose arguments point to static final fields of structurally immutable types (i.e. initialization-time constants, as opposed to compile-time). We allow handlers to declare the structurally immutable types they know (and care) about, rather than try to prove immutability as part of NullAway's analysis.

Note that a significant part of the code for implementing (2) involves threading knowledge of immutable types through the stateless AccessPath APIs. We accomplish this by creating a new `AccessPath.APContext` class, which is initialized by collecting information from all handlers whenever the dataflow analysis is initialized, and passed through all the operations manipulating access paths.

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Source Files on job #584.1