_stbt.android.AdbDevice: Refactor: Make `adb_device` a member not a property
_stbt.android.AdbDevice: Refactor: Make `adb_device` a member not a property

There's no need to re-calculate this each time we reference it.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester