class NaturalClientImpl

Sets up a callback to fire when the implicit ViewRef has gained or lost

focus. Typically, focus changes happen at a slow pace, such that every

transition is captured by the callback. However, focus changes can

happen rapidly, so the callback may "compress" focus change history into

just the latest known focus state.

The callback reports the last focus change spanning the duration from

its issuance to the previous callback's issuance. The client may observe

any of the following four transitions:

- true -> true: The ViewRef has lost focus at least once, and has gained

focus by the time of callback issuance.

- true -> false: The ViewRef has lost focus at least once, and has lost

focus by the time of callback issuance.

- false -> true: The ViewRef has gained focus at least once, and has

gained focus by the time of callback issuance.

- false -> false: The ViewRef has gained focus at least once, and has

lost focus by the time of callback issuance.

The client avoids "focus flapping" and always sees the latest state.

Every callback issuance is meaningful, and the client should renegotiate

focus-related services appropriately, such as IME.

Flow control: The caller is allowed at most one in-flight |Watch| call

at at time: subsequent |Watch| calls must wait until the acknowledgment

returns. Non-compliance results in channel closure.