Update semantics of on-body Keystore API.

Architectural changes from the Wear team have required changes to the
semantics of the Keystore on-body feature. Although no devices will
actually implement this feature until the release of new Wear devices
with the necessary sensors, we need to get the API documentation right
now.

Bug: 29369151
Change-Id: I857e00928f994a1337313123eae7e00534c53c3c