Results of ReadableState.read() should be snapshots of the underlying state

Future modification of state should not be reflected in previous calls to read(). For example:

@StateId("tag") BagState state;
Iterable ints = state.read();
state.add(17);
// ints should still be empty here.

Imported from Jira [BEAM-2975](https://issues.apache.org/jira/browse/BEAM-2975). Original Jira may contain additional context.
Reported by: millsd@google.com.

Start by locating the Java definition and implementations of ReadableState.read(), then inspect the BagState example described in the issue. Verify the current behavior and find the relevant state tests. Done means values returned by read() remain unchanged after later state.add() calls.

Written by the indexing model from the issue text.