jni: rename helper get() to get_raw()

Some java classes have get() functions that conflict otherwise
jni: fixup XBMCApp after get() rename
jni: have the compiler warn us about lots of potential badness

Mainly unused params. Since java params are passed through ..., it's easy to
miss one. An unused param in a native function is a really good sign that
something has been forgotten.
jni: use dummy casts to quash legitimate unused variable warning messages
jni: fix legitimate bugs seen with new warnings
fixed, these use CallVoidMethod, not CallObjectMethod