Install libaria2.pc only when libaria2 is enabled
Revert d50021d045df809aa8e6debf1a7e5727dabcd95c

It is safe to use prealloc for default, because of legacy fs (e.g.,
vfat) usage in Android.
Add DownloadHandle::getInfohash() and getPieceLength()
Make DownloadHandle::getNumPieces() return int