BinderProxy dump: distinguish between empty descriptor and dead remote.

getInterfaceDescriptor() will return an empty string when the call
to retrieve it into the remote process fails. Proxies to a dead binder
process then show up as having an empty descriptor in the proxy dumps,
and can't be distinguished from live proxies with an empty descriptor.

Instead, if we get an empty string, check if the proxy is still alive,
so we can more accurately dump. This will make it much easier to find
proxy leaks to dead binder nodes.

Bug: 109888955
Test: adb shell dumpsys activity binder-proxies
Change-Id: Ib37ec004f38c99743ffb4a62101c12626b649e5d