Consistently implement `toString()` in `BackOff` strategies

[sbrannen]

Overview

Currently, there are some inconsistencies in the toString() implementations in our BackOff strategies.

• FixedBackOff does not implement toString().
• FixedBackOffExecution implements toString() but claims that it is FixedBackOff.
• The existing toString() implementations wrap properties in {} instead of [].

Having proper, consistent toString() implementations in FixedBackOff and ExponentialBackOff is also a prerequisite for #35110, since the RetryPolicy will include the configured BackOff in its toString() representation.

Related Issues

• Redesign RetryPolicy to directly incorporate BackOff #35110