toolbox: reimplement watchprops using __system_property_foreach()

Internally, replace the watchlist array with a hashmap since the array
assumes properties are enumerated in a consistent order and foreach()
probably won't.  (find_nth() never guaranteed this either but it usually
worked in practice.)

Change-Id: I83843facdd671edd09652edf472e88ec3d1edd3b
Signed-off-by: Greg Hackmann <ghackmann@google.com>