[Ravenwood] Allow Ravenwood to use system-build.prop

To properly initialize default system property values, loading
build.prop built for the current generic build would be closest to what
running tests on a real device could be.

Flag: EXEMPT host test change only
Bug: 292141694
Test: $ANDROID_BUILD_TOP/frameworks/base/ravenwood/scripts/run-ravenwood-tests.sh
Change-Id: If7856266e6fde241db4d3df5b68cb3a626eeaea3