binder: A Channel might check its SecurityPolicy even after declaring termination

### What version of gRPC-Java are you using?
master/HEAD on any version of Android

### What did you expect to see?
Some reasoning/guarantee that a Channel will no longer call into its SecurityPolicy after it declares termination.

### Steps to reproduce the bug
One of many ways is to call `ManagedChannel.shutdownNow()` while it's establishing a new connection.

### Analysis
When using `AsyncSecurityPolicy`, `BinderClientTransport` calls `checkServerAuthorizationAsync()` while holding its `this` lock and only after carefully checking that it's not in state `SHUTDOWN` or `SHUTDOWN_TERMINATED`. This is safe because `clientTransportListener.transportTerminated()` is only called after transitioning into those states while holding the same lock. (We do this on the assumption that `AsyncSecurityPolicy` checks don't block)

However, when using an ordinary blocking `SecurityPolicy`, `checkServerAuthorizationAsync()` is implemented by `submit()`ing this presumably blocking work to `offloadExecutor`. We first do the same state checks, but because we release the `this` lock, the transport could shutdown and declare termination after this submission but before the offload executor actually calls `checkAuthorization()`.

Start by reading BinderClientTransport's AsyncSecurityPolicy path, especially checkServerAuthorizationAsync(), checkAuthorization(), shutdownNow(), and the transportTerminated() callback. Trace the offloadExecutor submission and state transitions to determine whether authorization can run after termination; done means the lifecycle guarantee is established and covered by a regression test or clearly documented reasoning.

Scritto dal modello di indicizzazione a partire dal testo della issue.