Ignore potential SystemProperties errors when setting net.dns

Test: as follows
    - built
    - flashed
    - booted
    - "runtest frameworks-net" passes
Bug: 33308258
Bug: 36249702

Change-Id: I76ae853ceb61b7b900f36f9c3cd67b2d2284aab0
(cherry picked from commit 4edba01e)