build: allow to set a consumer proguard file

This allow to specify a proguard file by defining LOCAL_CONSUMER_PROGUARD_FILE
that will inserted in the root directory of the aar and to be consumed by another apk
via gradle plugin

TICKET: RM-232
Change-Id: Ia3c11e5ea8e694800fb262b835432f86a6777f86
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
(cherry picked from commit 1efc554a)