screenrecord: allow recording without a time limit

Screen recording was limited to 3 minutes. The limitation
seems arbitrary so lets remove it.

Test: run `adb shell screenrecord --time-limit 0` and wait for  3 minutes and 1 second
Fixes: 237019907
Change-Id: Ic6710a123ef9fdf99a0f3df308937b8a33b9afcb