releasetools: Update the path to /default.prop for A/B devices.

On A/B devices (i.e. system_root_image="true"), /default.prop is
packaged at ROOT/default.prop (as opposed to BOOT/RAMDISK/default.prop
for non-A/B devices). Update the path so that we handle properties like
ro.bootimage.build.fingerprint properly.

The one for recovery is not affected, which stays at
BOOT/RAMDISK/default.prop for A/B devices and gets updated correctly.

Bug: 30811237
Test: Verify the property in the generated signed-TF.zip.
Change-Id: Id201a042d7ea988a64f89c6d04f43326a9851e27