fs_mgr: make is_device_secure() work even in the absence of properties.

During early mount property area is not initialized, and as a result an
'eng' build will always incorrectly be detected as a 'secure' build by
early mount code path resulting into verity error and consequent boot
loop.

The change here makes sure the is_device_secure() check works with /
without properties based on the 'eng' build based build flag so the
early mount code works fine both ways.

Bug: 35791581
Bug: 27805372

Test: Boot sailfish-{eng,userdebug} builds successfully w/ early
mount enabled

Change-Id: Icd101ccad56b669f49b60bbb3005d5be9f53b02b
Signed-off-by: Sandeep Patil <sspatil@google.com>