tools:releasetools:Use timestamp from build.prop while building system.img

We now use the timestamp written by the build system to build.prop as the
value passed to the make_ext4fs tool while generating the regular system
image. The ota package generation tools do it already. Without this change
the system image created by a normal make will never match the system image
in the target files package which would cause incremental block based ota's
to fail

Change-Id: I95c32c7ff9c2330a264853014a33811ca5ee3fe4