distinguish signed and unsigned user builds in UI

Add "dev-keys" or "test-keys" to the value of ro.build.display.id for
user builds.  (This is the property that is displayed under "Build
number" in the Settings UI.)  Modify the signing script to remove the
keys tag from this value when signing.

Change-Id: I3d9d92056f8567d7f84b1be047619be7c6e4c419