Only read system/build.prop for properties

otapreopt was trying to read /default.props in order to load default
properties. This file was removed some time ago causing otapreopt to
fail. This change makes it so we only attempt to read the
/system/build.prop file.

Test: Manual OTA of cuttlefish
Bug: 181182967
Change-Id: I8203868b7e68ba8bfd3b80897206a6be247cab92