chrome.hid-app: Rely on chrome's devices.list rather than CMD_PING

MP may not always be able to respond to CMD_PING, but may still be fully connected. MP client will now also know much sooner when the MP is unplugged.
Merge pull request #104 from jnwatts/feature/better_connection_handling_for_limpkin

chrome.hid-app: Rely on chrome's devices.list rather than CMD_PING