Cert: Introduce GD SL4A combination test infra

* GD SL4A combination test infra combines SL4A with GD
* It uses GD cert as a lower tester so that it can prepare any
  advertising packet
* It uses SL4A as the DUT so that it can repro any API level behaviors
* LeAdvancedScanningTest is the first example of such tests
* Has to override mobly host port in order to support ssh

Bug: 207525224
Test: m -j85 sl4a; adb install -r <path_to_sl4a_apk>; system/gd/cert/run --sl4a
Tag: #test
Change-Id: I32ed10052eb1864d7d4e8e69c6aaef5a7d0b5989