Load vendor public libraries to sphal namespace

Load vendor public libraries to sphal namespace
if it exists - preserve old behavior of loading
these libraries to default namespace if sphal
namespace is not present on the device.

Bug: http://b/37410104
Test: cts-tradefed run singleCommand cts --skip-preconditions -m CtsJniTestCases
      on marlin (with enabled sphal configuration) and on angler where ld.config.txt
      is not present.

Change-Id: Iaa3fa437ba2900acc2e5b9c78039fe1553e4c9dd