Don't dump binder proxies with the lock held.

There's still a path where the AMS lock is held when we're trying to
dump binder proxies; in this case, just don't dump any proxies at all,
since there's no safe way to do it with the AMS lock held.

Bug: 168353824
Test: adb shell am dump binder-proxies
Change-Id: Ia9b23821a953ac73fe6d67cc169998548680083d