Add canned permissions for */etc/build.prop

There were no canned permissions for product/etc/build.prop or
system_ext/etc/build.prop, which resulted in adb pushing over those
files getting 0666 permissions and then init silently rejecting them
has having insecure permissions.  Copy the permissions from
*/build.prop.

Bug: 375613290
Test: adb pull /product/etc/build.prop /tmp/product.build.prop &&
      adb push /tmp/product.build.prop /product/etc/build.prop &&
      adb shell ls -l /product/etc/build.prop
Change-Id: I0366e09244a72de2b532aba1b1c302e7b6f9d27c