Ignore serial number checks when emulating FBE.

When emulating FBE, we may have chmod 000'ed the directory to emulate
it being locked, which results in us failing to read the serial
number, causing a unintentional data wipe.

To avoid this, ignore the serial number check when emulating FBE,
which is a developer-only feature.

Bug: 29231715
Change-Id: Idb056ab378174004d1430b5e7c20db17de28231e