Fix memory leak in StrictMode violation throttling

* Add hashCode() to the base Violation class for dup detections
* Discard obsoleted violation fingerprints if possible

Bug: 159128771
Bug: 159626227
Test: run cts -m CtsOsTestCases -t android.os.cts.StrictModeTest
Test: Manual - Loop generation of StrictMode violations
Test: Manual - Check heap dump
Change-Id: I19a0922fe010fad97b6b819e73acb7db08f84930