Add new version of startUser performance test

Similar to what was done for switchUser, this commit adds a new version
of the startUser test. In this variation, a wait for cool down was added
in between iterations.

Bug: 261621873
Test: atest UserLifecycleTests#startUser_realistic
Change-Id: Ia4ff7ed2df7b209175185794eac0cb17630ddc9e