trusty: Write out sancov file when fuzzer exits

Add emission of sancov file when CoverageRecord is destroyed. This
will occur when a fuzzer driver exits cleanly, i.e. -runs=0 with an
existing corpus.

Test: make trusty_gatekeeper_fuzzer
Test: adb shell ./trusty_gatekeeper_fuzzer -runs=0 corpus
Bug: 175221942
Change-Id: I6bd1c8b2f2091e894c35f7a4874b54577a91c8fc