releasetools: Remove the reloading of target info dict.

In non-A/B OTA path, we've been loading the info dict for the target
build twice (once to have a peek at the OTA type, with a reload after
unzipping the target-files zip). Remove the reloading.

This CL moves the loading of the source info dict up a bit to avoid
having two copies in the A/B and non-A/B paths. It also moves up the
sanity checks of the loaded info dicts before unzipping the target-files
zips.

Test: Generate full and incremental packages for bullhead and marlin.
Change-Id: Iccb953feb0b6ecf62e6f73d6e9ceea00077d098d