Remote: Fix an issue that a failed action could lead to RuntimeException caused by InterruptedException thrown when acquiring gRPC connections. https://github.com/bazelbuild/bazel/issues/13239

When --keep_going is not enabled, Bazel will cancel other executing actions if an action failed. An action which is executing remotely could in the state of waiting for a lock available to acquire the gRPC connection. SharedConnectionFactory uses ReentrantLock#lockInterruptibly to acquire the lock and will throw InterruptedException when the thread is interrupted which happens when the action is cancelled by Bazel. However, this InterruptedException is wrapped inside a RuntimeException results in a build error.

ReentrantLock was choosen initially to implement a hand-over-hand locking algorithem but it's no longer necessary after a few iterations. This change replaces ReentrantLock with `synchronized` keyword so we won't throw InterruptedException when acquiring gRPC connections. Call sites can still throw InterruptedException to cancel an action execution.

PiperOrigin-RevId: 365170212