Call messages delivered to the NSE have to be handled in the main app,
so they're put in a synced key-value store and the app is notified.
Rings are processed oldest-to-newest in case you get two calls in
succession before the main app awakes. However, if this processing is
interrupted, the store won't get cleared, and the app may try to
process the *old* ring when it's woken up by a *new* ring. This would
*still* be correct (e.g. for showing missed call notifications) except
that the "age" of a ring message is calculated purely on when it makes
it to the user's device, since it was assumed that it would be
processed promptly. Since this is not necessarily the case in these
error scenarios, this commit adds a second timestamp to track the skew
between when the NSE enqueues the call message and when the main app
processes it.
a302e7cfbc