Update PropertyWatcher to wait for property create

Previously, PropertyWatcher would only wait for changes to a property,
however, if a property had not yet been created then PropertyWatcher
would fail. With this change, PropertyWatcher::wait will wait for
properties to be created as well as changed.

Bug: 172013262
Test: atest system/keystore/keystore2
Test: statsd_testdrive 10103
Change-Id: Ic2759581459759738c11e0c452c1457a4a95feea
1 file changed