SL4A: Throw exception when test fails and clear paired devices

* Return False no longer makes the test fail in mobly
  we should raise exception instead
* Clear paired devices before tests so that they do not interfer with
  later tests

Bug: 232048916
Tag: #test
Test: gd/cert/run --sl4a_sl4a --clean
Change-Id: Id1e4cb30e28a13203848cb5c406ecb784fe2315c