Read the session deadline under the status read lock (#7550)

GetSessionExpiresAt took the exclusive lock for a plain field read, so
every caller queued behind writers and behind each other. The Android
SessionMonitor polls it from the main thread, and in the captured ANR
that is exactly where the main thread was blocked while hundreds of
peer-list callbacks held or waited on the same mutex.

d.mux is already an RWMutex and the other getters use RLock; this brings
the deadline read in line with them.

client/internal/peer/status.go

@@ -819,8 +819,8 @@ func (d *Status) SetSessionExpiresAt(deadline time.Time) {
 // "none" would blank the UI at the exact moment it should say the session
 // ended.
 func (d *Status) GetSessionExpiresAt() time.Time {
-	d.mux.Lock()
-	defer d.mux.Unlock()
+	d.mux.RLock()
+	defer d.mux.RUnlock()
 	return d.sessionExpiresAt
 }