bionic: add __system_property_foreach

find_nth() will be inefficient on a trie.  Since find_nth() is only used
internally and only for enumerating properties, we can add a foreach()
function to do this directly.

Signed-off-by: Greg Hackmann <ghackmann@google.com>

(cherry picked from commit 57741840)

Change-Id: Iaca97d1182ce2c28863ba85241cbb5cf6185eb2f