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
9289a04315