Allow sensors bleed all remaining events before test starts

After framework shutdown, there may be stall events when sensors vts
target test starts. These events make vts flaky. This CL allows the
test to remove all those events to avoid test results being affected.

Test can run repeatedly > 20 times without failure with this patch.

Test: vts-tradefed run commandAndExit vts
       --module SensorsHidlTargetTest
Bug: 33618936

Change-Id: I027e88b2b7425136543b15cf39afb203c315d8bf