Add support for O-MR1 non-A/B images

prop.default file is located at /etc/prop.default on non-A/B images. To
support both non-A/B and A/B images, this script now tries both
/etc/prop.default and /system/etc/prop.default.

Bug: N/A
Test: try to patch both aosp_arm64_a image and aosp_arm64_ab image with
"-v 8.1.0" option.

Change-Id: If012b209d45fc856c10cc33e4fa837dc99bfa162