Simplify settings for library predexing

I thought the code handling predexing for the second parameter of `dxOpts` was not very straightforward. I replaced it with my own setting key toggling predexing for every library input.

Basically, if `predexLibraries` is `false`, then run `dxTask` as before, else predex every `file.jar` input as a `file.jar.apk` predexed cache that won't be rebuilt unless the origin JAR has changed, then link them to the compiled classes.