test_vendor: Allow includes from bt_hci_defs

Avoid duplicating header files for BT constants.

Update Android.mk to search $(BT_DIR)/include and
$(BOARD_BLUETOOTH_BDROID_BUILDCFG_INCLUDE_DIR)

Change-Id: I1403724c1cf9cda8156a5b65cf55da8178bd910f
Signed-off-by: Myles Watson <mylesgw@google.com>