EventLogHelper: Clean up code

Use scoped wrappers and unique_ptr.

Bug: 67511924
Test: m
Test: Device boots
Test: cts-tradefed run commandAndExit cts -m CtsUtilTestCases -t android.util.cts.EventLogTest
Change-Id: If9abfb8b43a4653ccd7fc553257ab3529063577f