The Android browser does not support websockets which is a requirement for noVNC.
I am guessing you are connecting to vnc_auto.html, if you connect to vnc.html instead you will get a control bar at the top of the screen. In the control bar there is a button for bringing up the on-screen keyboard.
Note that prior to a fix which was applied to noVNC today the on-screen keyboard did not work in Chrome on Android.