fs_mgr: refactor: consolidate device tree file reading in one place

If Device tree values are read for comparison, they produce false
negatives with std::strings due to trailing '\0'. This change
consolidates the triming of trailing null into a single helper function
to be used everywhere fs_mgr reads DT values for comparison or other
reasons where it wishes to have the trailing null trimmed.

b/27805372

Test: Boot sailfish w/ early mount /vendor

Change-Id: If71efc830dc440323df764c7461867e71ed6515b
Signed-off-by: Sandeep Patil <sspatil@google.com>