SurfaceFlinger: Validate layers before casting.

Reinterpret casting random IBinder = no-fun. I first attempted
to use inheritance of "getInterfaceDescriptor" in Layer::Handle but
departing from "standard-layout" (e.g. using virtual methods) means that
downcasting with static/reinterpret_cast is no longer valid. Instead I opted
for the pattern the system-server uses of maintaing a map.

Bug: 129768960
Test: InvalidHandles_test.cpp
Change-Id: Ica8d08a2daf5e80d187e818adbff0c7901db69f7