Report errors from CtsTestServer for improved debugging

Switch from using ExecutionService.submit() to Executor.execute()
because the Future was never being checked. Switched to using
a Runnable, because no value is returned. Added logging for
any exception generated.

Bug: 21874093
(cherry-picked from commit a1811d4ec0cb3f8d5cdb8a9bf745cec4458f51c5)

Change-Id: I23d3a770929372e3b234a0c4577d519bfefdaacc