Keystore2: fix test_large_number_of_concurrent_db_manipulations

The test was disabled and got stale. Fix the test so it uses the GC,
as it's useful for checking perf-related code changes. Will investigate
fully re-enabling the test on T.

Bug: 190142197
Test: keystore2_test
Change-Id: Ifc0a4a5b3c8c301c42d068ee46754d877eeb10bc
Merged-In: Ifc0a4a5b3c8c301c42d068ee46754d877eeb10bc
1 file changed