Use the same FindClass argument for Android and regular Java

Its unclear whether the android variant (no L prefix, ; suffix) was
ever tested on normal Java or not, but the existing code generates
a large volumes of warnings on OpenJDK 11 like:

WARNING in native method: JNI FindClass received a bad class descriptor "Lorg/ldk/impl/bindings$LDKEvent$PaymentReceived;".  A correct class descriptor has no leading "L" or trailing ";".  Incorrect descriptors will not be accepted in future releases.