Add separate clear method for audit log callback

Currently clients have to provide an executor even when callback is null
which doesn't make any sense. With this CL both executor and callback
are non-nullable, and instead a separate clearing method is added.

+ also add 'throw' before rethrowFromSystemServer

Bug: 295324350
Test: atest CtsDevicePolicyTestCases:SecurityLoggingTest
Change-Id: I2262a7c2d8d433220bca125570aa04dd5c1c7df7