vendor_libs: create Android.mk

all-subdir-makefiles call works for only one
nested sublevels, so we need to add a default
Android.mk in order to be able to build libbt-vendor module.

BUG=28014054

Change-Id: I7cc004d2a4dd6216a80d13c6384d9dcde328323e
Signed-off-by: Cristian Iorga <cristian.iorga@intel.com>