class SetActiveChannels

Sets which channels are active via a bitmask.

The total number of channels is the `number_of_channels` in `Format`, specifically in

`PcmFormat`, i.e. this bitmask has up to `number_of_channels` bits set (maximum 64).

The least significant bit corresponds to channel index 0. Channels not set (bits are 0) in

the bitmask are inactive. By default all channels are active. Hence creating a RingBuffer

turns on the hardware associated for all channels.

Inactive channels indicate to the driver that it may turn off hardware associated with the

inactive channels. A subsequent `SetActiveChannels` setting an inactive channel to active

may incur in a `turn_on_delay` to actually restart playback/capture of the channels.

Deactivating one, several, or all channels does not `Stop` the ring buffer, nor does it

change the ring buffer's behavior with regard to position. Once `Start` is called, a ring

buffer's position advances (and position notifications sent as needed) regardless of the

number of active channels, including if no channels are active. This means that the format

in the ring buffer is not changed.

If the driver does not support deactivating channels, it must return `ZX_ERR_NOT_SUPPORTED`.

If the mask is incorrect, i.e. enables channels outside the number of bits to use for a

given `number_of_channels`, then the driver must return `ZX_ERR_INVALID_ARGS`.

The `set_time` value (in the CLOCK_MONOTONIC timeline) indicates when configuring

the hardware to activate or deactivate channels is completed. `set_time` does not include

the potential `turn_on_delay`, the driver does not delay the reply waiting for the

hardware to actually turn on, the driver replies with a `set_time` indicating when the

hardware configuration was completed. If the requested channel configuration is already

active, the returned `set_time` can be before `SetActiveChannels` was called but must be

before the reply is sent. If called again with the same configuration, the reply must

include the same `set_time` value as was previously returned.

For input channels, it is not required that the driver zero-out inactive channels.

If `SetActiveChannels` is called for a second time before the first call has completed,

the channel must be closed with an error `ZX_ERR_BAD_STATE` returned.