This update renames an internal mail calling field to make it clear when a participant has muted themselves. It prepares the system to distinguish self-muting from other muted states in future call features, with no expected change for users today.

This commit renames the isMuted variable into isSelfMuted to reflect the
user action of self muting, as opposed to the state of being mute,
which will be distinct in an upcoming commit.

part of task-2720026