Always remove the observers in cancel to avoid object leakage

We didn't remove the observers if the cancel invocation came from end(),
this caused leakage of FrameTracker, so we always remove the observers
no matter where the cancel invocation comes from.

Bug: 181683700
Test: atest FrameworksCoreTests:InteractionJankMonitorTest
FrameworksCoreTests: FrameTrackerTest
Change-Id: Ia03ebc3b579257dd387eaa1be65b9d22e7b7bf91