update ro.build.tags when signing release builds

Apply the same changes to tags to ro.build.tags that we do for the
tags in the fingerprint (ro.build.fingerprint) and the description
(ro.build.description).

Change-Id: Ie5a057d8f04cbc32d849f91e1f9d2ea7832e81f6
http://b/2363735 - release-key user builds ship with property ro.build.tags == test-keys