bluealsa plugin: monitor the state of service via dbus

By monitoring the changes of names on dbus, and
by asking dbus for the list of available names at startup,
the bluealsa plugin can now be started at whatever time,
and is also robust to bluealsa restarts.
Moreover, since the bluealsa name has the HCI interface
has a suffix (org.bluez-alsa.hci0, for instance), there
is not need to hardcode the hci name.

Even better, the plugin supports several instances of bluealsa
(since the daemon needs one instace per hci device), and will
add a watch for added transports for every instance it detects.

Complete cleanup of the created streams in softmixer is done
when one watched instance disappears.

Finnaly, there is a new cmd line '-DWITHOUT_BLUEALSA' option to CMake,
for developpers that want to use 4A on host, without the bluealsa
library. Default build (SDK, bitbake) consider that the bluealsa
shared library is always available.
Notice that in the future, the bluealsa library might disappeared,
to be replaced by a full dbus interface to bluealsa.

BUG-AGL:SPEC-2126

Change-Id: I67fc6edf5147c71dc97f1fc1257d3dadbdde20fc
Signed-off-by: Thierry Bultel <thierry.bultel@iot.bzh>

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