Prevent autobinding ProviderWatcher abuses

With this change, the watcher will no longer bind immediately after
(re)starting the provider proxies. Instead, binding will happen
during the first scanning request after that.

Bug: b/263520343
Test: atest MediaRouter2HostSideTest
Change-Id: I752fe0fecaef1086fcee2e6fa61e2669dd7fefc5