adb: initialize mDNS asynchronously.

Use fdevent_run_on_main_thread to initialize mDNS in a thread and
register an fdevent from the main thread upon success.

This reduces the startup time of `adb server` by ~3 seconds when mDNS
can't be successfully started. With an already running adb server,
`time adb server nodaemon` goes from:

    adb server nodaemon  0.00s user 0.16s system 4% cpu 3.817 total

to:

    adb server nodaemon  0.00s user 0.01s system 1% cpu 0.665 total

Bug: http://b/37869663
Test: `adb server nodaemon` with an existing adb server
Change-Id: Ia5a1a2a138610f3bf6792400050ca68f95ae3734

adb/transport_mdns.cpp

@@ -24,6 +24,8 @@
 #include <arpa/inet.h>
 #endif
 
+#include <thread>
+
 #include <android-base/stringprintf.h>
 #include <dns_sd.h>
 
@@ -262,19 +264,22 @@ static void DNSSD_API register_mdns_transport(DNSServiceRef sdRef,
     }
 }
 
-void init_mdns_transport_discovery(void) {
-    DNSServiceErrorType errorCode =
-        DNSServiceBrowse(&service_ref, 0, 0, kADBServiceType, nullptr,
-                         register_mdns_transport, nullptr);
+void init_mdns_transport_discovery_thread(void) {
+    DNSServiceErrorType errorCode = DNSServiceBrowse(&service_ref, 0, 0, kADBServiceType, nullptr,
+                                                     register_mdns_transport, nullptr);
 
     if (errorCode != kDNSServiceErr_NoError) {
         D("Got %d initiating mDNS browse.", errorCode);
         return;
     }
 
-    fdevent_install(&service_ref_fde,
-                    adb_DNSServiceRefSockFD(service_ref),
-                    pump_service_ref,
-                    &service_ref);
-    fdevent_set(&service_ref_fde, FDE_READ);
+    fdevent_run_on_main_thread([]() {
+        fdevent_install(&service_ref_fde, adb_DNSServiceRefSockFD(service_ref), pump_service_ref,
+                        &service_ref);
+        fdevent_set(&service_ref_fde, FDE_READ);
+    });
+}
+
+void init_mdns_transport_discovery(void) {
+    std::thread(init_mdns_transport_discovery_thread).detach();
 }