adb: Make the Connection object a std::shared_ptr

This change is in preparation to allow the TCP-based transports to be
able to reconnect. This is needed because multiple threads can access
the Connection object. It used to be safe to do because one instance of
atransport would have the same Connection instance throughout its
lifetime, but now it is possible to replace the Connection instance,
which could cause threads that were attempting to Write to an
atransport* to use-after-free the Connection instance.

Bug: 74411879
Test: system/core/adb/test_adb.py
Change-Id: I4f092be11b2095088a9a9de2c0386086814d37ce