Update git submodules

* Update system/bt from branch 'master'
  to 4cc2f587357b28985ae1910174a6b5b8f6654a25
  - Merge "Use mobly.asserts"
  - Use mobly.asserts
    
    Test: cert/run_cert_facade_only.sh
    Change-Id: I3f19bba39953eca9c013aab8813610c3fbfb5f83
    
1 file changed