Add flow control for PAN.

This change reads from the TAP fd only when btpan_cb.flow == 1.
The value of this flag changes based on L2CAP congestion events.

http://b/13246629

Change-Id: Ic28bf65737dbd74c41b68198fceee58e3bff08d4

Conflicts:
	btif/src/btif_pan.c
3 files changed