Allow overlap in UidMarkMap

To support simultaneous tuns UidMarkMap now allows for
overlaping/duplicate rules. If there are multiple rules for a given uid
the most recently added rule will be used in all cases.

When overlapping rules are added in addUidRule there may be multiple
iptables rules matching the uid. Since addUidRule appends it will use
the most recent rule as well, no change required. Previously
UidMarkMap->add would fail and the rule would never be added.

Bug: 12134439
Change-Id: I5f2976dd3ee334584a9f98f6eacd5edbe5c9bb6b