Your subtitle (
.sub + .idx) is a graphics based subtitle and I frankly don't know if (and also have serious doubts that) BSPlayer Lite for Android can show those subs as a native feature.
At the moment I don't have any idx/sub-files, so that I cannot test.
However, with respect to text-based subtitles
(you can check on your PC if they are readable in e.g. MS-Notebook/Notepad: I havn't found an application with similar possibilities for Android yet, but then: I sofar didn't seek one) such as .srt (but here also are text-based .sub-files!) I'm sure they can be played using BSPlayer's native feature to play subtitles.
But why don't you use BSPlayer Lite's feature to automatically search and download on-line subtitles?