Dump the unused IPv4 BPF rule age correctly

Print a dash for the unused "lastUsed" of the rule.

The age should not equal the bootime if the rule has never been
updated the "lastUsed" field.

Format:
[inDstMac] iif(iface) src -> nat -> dst [outDstMac] age
[00:00:00:00:00:00] 14(rmnet0) 140.112.8.116:443
-> 30(30) 10.170.56.233:43720
-> 192.168.45.236:43720 [be:34:40:28:33:5f] -

Bug: 190783768
Test: dumpsys tethering
Original-Change: https://android-review.googlesource.com/1751022
Merged-In: Ie771becd2f72518cf02a86e5ae228315785752a5
Change-Id: Ie771becd2f72518cf02a86e5ae228315785752a5
1 file changed