adb: add lock to remove_socket.

The comment that was previously here says that local_socket_list_lock
must be taken, but this function is exposed to external callers that
can't possibly take the lock.

Bug: http://b/65419665
Bug: 64709603 (presubmit balking at the line above)
Test: python test_device.py
Change-Id: I12d464933936b2a210a827ccf19ea201020d8d78
(cherry picked from commit 62c92f0c05)

adb/sockets.cpp

@@ -96,7 +96,7 @@ void install_local_socket(asocket* s) {
 }
 
 void remove_socket(asocket* s) {
-    // socket_list_lock should already be held
+    std::lock_guard<std::recursive_mutex> lock(local_socket_list_lock);
     if (s->prev && s->next) {
         s->prev->next = s->next;
         s->next->prev = s->prev;