properties: do not fail in the absence of vendor properties

/vendor is not mounted in recovery and so the property initialization
must not fail if the vendor property context is not found.

Bug: 38212419
Test: boot into recovery, mount system and run adb shell getprop

Change-Id: I9f7c2fe2b20da86194eff8e7bf94b2352e50bf27
Signed-off-by: Sandeep Patil <sspatil@google.com>