Fix MonitorContendedEntered test.

This test was incorrect and slightly flaky due to small differences in
the timing of events from what the test expects. This fixes the test
to instead use MonitorInfo commands to ensure that the debuggee is in
the expected state.

Test: manual
Test: ./art/tools/run-libjdwp-tests.sh --mode=host
Bug: 142039794
Change-Id: Ied8adb815d49f593d5df45e338e9a8c08b292df8
3 files changed