Change clearEvents() to clear existing events rather than setting

mEvents to null to remove possible NPE.

* clearEvents() needs to be called by destroy() which could cause race
condition on whether mEvents is null for other func calls.

Fixes: 185162720
Test: atest CtsContentCaptureServiceTestCases
Change-Id: I7330e651bc890c8a86822158efcf17a61e927112