mobile/sensor: don't panic, return an availability error on non-android

Change-Id: I42152d6fd5af220a780a6e3a288a97551ee53f39
Reviewed-on: https://go-review.googlesource.com/8413
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

sensor/sensor_notandroid.go

@@ -6,27 +6,29 @@
 
 package sensor
 
-import "time"
+import (
+	"errors"
+	"time"
+)
 
 type manager struct {
 }
 
 func (m *manager) initialize() {
-	panic("not implemented")
 }
 
 func (m *manager) enable(t Type, delay time.Duration) error {
-	panic("not implemented")
+	return errors.New("sensor: no sensors available")
 }
 
 func (m *manager) disable(t Type) error {
-	panic("not implemented")
+	return errors.New("sensor: no sensors available")
 }
 
 func (m *manager) read(e []Event) (n int, err error) {
-	panic("not implemented")
+	return 0, errors.New("sensor: no sensor data available")
 }
 
 func (m *manager) close() error {
-	panic("not implemented")
+	return nil
 }