Screen flashes once then become black with proximity sensor covered after outgoing call rejected.

[KasperLau]

current build number: 274
device name: krillin
channel: ubuntu-touch/rc-proposed/bq-aquaris.en
last update: 2016-03-07 02:17:57
version version: 274
version ubuntu: 20160305
version device: 20160108-efc96d8
version custom: 20160111-926-36--vivid

Steps:
1. Make a outgoing call in Dialer-app.
2. Keep covering the proximity sensor.
3. Reject the call from another phone.
4. Check phone's behavior.
5. Uncover/cover the proximity sensor several times.
6. Check phone's behavior.

Actual result:
Step4. Screen flashes once then become black.
Step6. The screen will turn on/off, seems that DUT is still in calling mode.

Expected result:
Step4. Screen should turn on.
Step6. Nothing happened if uncover/cover the proximity sensor.