class WireSyncBufferClientImpl

Sent to the registered client when the memory pressure level changes.

`level`: indicates the current memory pressure level.

Will also be invoked on initial connection via `RegisterWatcher`, so that a newly

registered client can discover the current memory pressure level.

The watcher must immediately reply with a message to acknowledge that it has

received the level change notification, and has initiated required actions as a

result. It may then continue to reclaim memory asynchronously after sending

the acknowledgement.

Some helpful guidelines for clients:

1. The watcher will be notified of new pressure level changes only after a reply

corresponding to the previous message has been received by the provider.

If multiple level transitions occur during that time, the watcher will be

notified of the latest pressure level.

2. The level changes are edge-triggered, and clients are expected to maintain

local state to track the current pressure level, if required. For example,

a job might be notified of a CRITICAL level and drop all its caches as a result.

Some time after this, it might want to trigger an activity that causes a

fair amount of memory to be allocated. At this point, the job is expected to

remember that the last pressure level it saw was CRITICAL, and refrain from

triggering the memory-intensive activity.

3. As a performance optimization, the provider may decide to skip sending

messages for some pressure level changes. For example, when oscillating across

the NORMAL / WARNING boundary, it might not be worth notifying clients of every

single transition. The provider might rate-limit messages in this case.

On a similar note, the provider may decide to send repeated messages at the

same pressure level, particularly CRITICAL, to indicate that further action

needs to be taken.

Caller provides the backing storage for FIDL message via an argument to `.buffer()`.