Fix potential null error in clearEvents.

* This is follow up from previous patch to fix an unresolved
comment.

* We're still changing the clearEvents mechanic to not null the list but
actually clear the list of events. We also change when the
flush->clearEvents call happen during the destroy process so it occurs
on the handlerThread when each session does its own cleanup.

Bug: 185162720
Test: atest CtsContentCaptureServiceTestCases
Change-Id: Ide5490903deec8c4bc8aae5dc76b1fadca29dbd0