fs_mgr: Don't run e2fsck inside SDK system images.

These images do not have GPL-ed binaries like /system/bin/e2fsck so
avoid running the program when we detect that we're running inside
one of them.

Note that this does not affect other emulator-based build products
(e..g full-eng instead of sdk-eng), which do have the binaries.

BUG=13057123

Change-Id: Ia42f1d02a3845fbf4b2f9d95818f35d760711a12