Fail idmap parsing earlier if version differs

Variable length strings are now in the idmap format. Idmap attempts to
parse the old idmap files using the new format and can interpret old
data as the length of a variable length string.

Fail parsing idmap header early if the idmap magic or version differs
from what is expected.

Bug: 177295273
Test: boot, apply change, reboot
Change-Id: I2914f11547424457fdb78de0ad03976e1fc81084