Chang flash task to take in source

Old changes didn't actually take into account sources. fastboot update
{update.zip} should read fastboot-info from the update package. It was
instead flashing local images + reading local fastboot-info.txt

Test: m fastboot, fastboot update, fastboot flashall
Bug: 283330320

Change-Id: I9587a7735ddfbfb661153eb12ebc3caa7c32f141