VICTORY: stage1: Update install_zip check.

Rely on the existence of the variable instead of the
actual file due to a potential difference in path
(/sdcard vs /storage/sdcard0, etc).

Fixes the auto-resume of an install after rebooting
from a kernel upgrade due to partition resizing.

Change-Id: Ia50ed7154d185c9db7a465c048f4eeaacc3f23f9
Merge "VICTORY: stage1: Update install_zip check." into cm-11.0