sign_target_files_apks: rewrite build.prop correctly in eng builds

In eng builds, ro.display.id has many space separated items and was
resulting in an error when trying to rewrite it as 'value' gets
turned into a list and never converted back to a string.

Change-Id: I6c8633ed2eb52c56a4097992a32d53d80df4f844
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>