RESTRICT AUTOMERGE Make log reader thread a class member

pushedEventThread references class members after detaching. Making
pushedEventThread as class member and joining in statsService
destructor. Adding a method to stop readLogs thread.

Ignore-AOSP-First: Bug is in still security triage and fuzzer is
crashing on startup.
Test: atest statsd_test
Test: m statsd_service_fuzzer && adb sync data && adb shell
/data/fuzz/arm64/statsd_service_fuzzer/statsd_service_fuzzer -runs=10000
Bug: 285645039

Change-Id: I1e886f9ccb7203714216da061c35e793b2a63d8a