android/AVDTP: Make signalling channel priority 6

This makes signalling priority 6 so it can push commands before the
stream channel, without this the stream channel may be schedule
first and cause the signalling commands to timeout while waiting a slot.
android/AVDTP: Make stream channel priority 5

This makes channel priority 5 so it has higher priority than regular
traffic but less than signalling channel.
android/hal-audio: Fix not handling EINTR errors

If the kernel interrupts us while writting just try again.
android/hal-audio: Set stream fd to blocking

This makes the stream to block on io operation so it does not return
EAGAIN on syscall such as write.
[travis-ci] Add configuration file
android: Fix GNU old-style struct field designator

This gives a warning on clang, that turns into an error due to -Werror.
attrib-server: Remove unnecessary fields from struct gatt_channel

These fields are only used internally by attrib_channel_attach(). The
"No GATT server found..." error message was also removed as it is also
displayed by find_gatt_server().
core: Fix associating a GAttrib to a device on incoming connections

If BlueZ is running as slave, incoming connections using ATT CID will
have a GAttrib created for them. But this GAttrib will not be assigned
to the underlying device (whose struct btd_device instance was created
in response to the Device Connected mgmt event).

This patch fixes this by assigning the GAttrib to the btd_device as soon
as a connection is first requested, which is done using attio callbacks.

Note that a new GAttrib reference is taken because attio callbacks "own"
device->attrib and thus need their own reference.
alert: Only remove attio callback after ATT request was sent

If there is a single registered attio callback, any pending ATT requests
will be dropped as soon as it is removed. This commit makes sure that
the request is sent before removing the callback.

Other places using attio callbacks may need fixing as well.