Use new netlink attribute for scan result timestamp

Use new netlink attribute for scan result timestamp

This CL prioritize the new netlink attribute
NL80211_BSS_LAST_SEEN_BOOTTIME to the old NL80211_BSS_TSF
because if driver supports the new attribute, the new
attribute will carry more accurate data. If not, we still
can fall back to NL80211_BSS_TSF.

Bug: 37758502
Test: compile, unit tests, integration test
Test: manual tests:
1. Test devices which have new kernel/driver supporting this attribute.
Test that most APs still consistantly show up in scan result.
1. Test devices which don't support this attribute. Insert logs to test
that wificond can fall back to the old attributes.

Change-Id: I14226b77e8bf50f4e035679fff3313affb800e86