[bluez] Defer A2DP acquire for a while if there's an ongoing call

Pulseaudio may get the notification that a call has ended
significantly earlier than bluetoothd. This means that it can try to
acquire A2DP transport before bluetoothd has notified a connected
headset that call/callsetup has ended. At least Sony SBH50 has
difficulties dealing with AVDTP stream start if it thinks callsetup is
still in progress.

Work around the problem by delaying A2DP acquire by up to 2.5 seconds
if there is an ongoing call when the D-Bus request comes in.