Don't hold sProxyLock while retrieving interface descriptors.

Since we can't guarantee apps won't call back into us while calling
getInterfaceDescriptor() (even though that's a *really* bad idea),
don't hold any locks while doing so, to protect us from deadlocks.

Bug: 140603195
Test: adb shell am dump binder-proxies
Change-Id: I6289c799aa7027f80792cb751fe1dc437552ea58