Try to avoid IndexOutOfBoundsException.

Add a limit to the number of dropped constraints and stop modifying the
number of required and dropped constraints independently to prevent the
code from creating invalid indices at random times. Also fix some of the
tests and make them more robust.

Bug: 253237201
Test: atest --rerun-until-failure 25 CtsJobSchedulerTestCases:FlexibilityConstraintTest
Test: atest --rerun-until-failure 25 FrameworksMockingServicesTests:FlexibilityControllerTest
Change-Id: I072da5c6271eecc3e8a7e3983f414c752d40cba8