Tried with:
-i /sdcard/okobot/entrada.mp4 -i /sdcard/okobot/back_res.png -strict experimental -filter_complex "overlay=0:0" -y /sdcard/okobot/salida.mp4
(Using escape " to do quotes in string).
Gives error: [AVFilterGraph @ 0xb9567270] No such filter: '"overlay'
(Take attention to the quotes here).
FFMPEG can't use single quotes to work. There's workaround also without use quotes, but doesn't work for multiple -filter_complex combinations (forces the use of double quotes).