Merge event-log-tags from platform.zip with local *.logtags

PDK fusion builds will include system/etc/event-log-tags from a full
build from platform.zip, which needs to be merged into the event-log-tags
built locally based on *.logtags from projects that have source.

Bug: 6297675
Change-Id: I91409f37c6dc88d6942eade2946155c2a6640b6f