apps: logprocessor: Fix tracking of log starts

Shill's WiFi logs record scan start events as:
  "Scan on $DEV from ..."

Since logprocessor's regex for scan starts does not correspond with
this format, following a shill change in mid-2016, logprocessor's
functionality for tracking scans is somewhat broken and has been for a
while. This change fixes the regex so that logprocessor can properly
track scan starts and ends.

BUG=None.
TEST=Manual testing with log containing scans.

Change-Id: Ifdffc126fff9126a609393f2895b6c95e2cddbe7
1 file changed