[cts] fix flaky InstallSessionCleanupTest

Due to duplicated test threads or repeated launching events, there could be multiple times of NUM_NEW_SESSIONS at the time the assertion is called. Change the test condition to be more inclusive.

Change-Id: I213791607ed2f5fa423194cbe903987cd0bf8493
Test: could not reproduce but theoretically this CL should fix it
BUG: 268221621