PX4 Firmware plugin: First try to switch flight mode, then arm if successful - which is a lot safer than arming and then seeing if we can switch modes.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Just use settings defaults

Fixed found bugs

Plus low battery announce changes

Previous version would require plugin to override all the
isFooFlightMode methods to work correctly. Now just change mode name
and regular plugin code works correctly.