Generate DBus proxies and adaptors separately

Enable daemons exposing an interface over DBus to easily
build client libraries.  Now daemons can write rules like:

include $(CLEAR_VARS)
LOCAL_MODULE := libdbus-binding-example-client
LOCAL_DBUS_PROXY_PREFIX := dbus-example-example
LOCAL_SRC_FILES := \
    dbus_bindings/org.chromium.Example.Manager.dbus.xml \
    dbus_bindings/dbus-service-config.json
include $(BUILD_SHARED_LIBRARY)

to expose a client library.

While here, add support for generating independent adaptor header
files on a per interface basis.

Bug: 22608897
Change-Id: I011f9afc234811c31e445898321c2731c482fa77