Use a non-existing PID for cleanup

9999 could be a valid PID and pass the cleanup during test,
the max number should be bound by 32,768 usually or at max
4 million.

Bug: 363972064
Flag: TEST_ONLY
Test: atest PerformanceHintTests
Change-Id: I9465eac3f926a7fa3d20c4782a77d0d72a6445d5