Hold only notification info in VoipCallMonitor.

Currently we're holding the instances of NotificationStatusBar in
VoipCallMonitor to track the notification of ongoing voip call. This
cause leaks of notification. Fix this by only track the necessary
information of the notification.

Bug: 280787082
Test: atest VoipCallMonitorTest
Change-Id: I151ececc22466bc07e083cd83d895dbdd88b8e12