First of, my Lightroom (5.2) does not import
.thumbnails or any other hidden folder. This might be Mac specific, as files starting with dot are hidden in Unix, but not in Windows.
There isn't a good option to filter out folders from import. What you could do is to import thumbnails too, and filter those out. At least with my Android phone (Nexus 5), thumbnails does not include exif metadata, which is enough to filter those for deletion.
Of course, importing and then deleting takes additional disk space (not too much; my
.thumbnails have over 6000 files and is just 300MB). Also, having to filter and delete thumbnails after each import is cumbersome.
Here's alternative approach for Mac users (and for Windows users too, after installing additional programs): open terminal (Spotlight->Terminal, enter) and run:
rsync -a /Location/of/Android/DCIM --exclude .thumbnails ~/Pictures/Android_photos
Replace file paths accordingly. This will sync all files, except
.thumbnails folder to Pictures. After that, in Lightroom import, select "Add" from top of the dialog, instead of Copy.