Is it possible to generate user-defined signal using an android phone?
Theoretically it may be possible to do something like this, but you'd need a lot of proprietary information from the manufacturer.

The part of the system that controls what gets output on the radio is known as the baseband processor; it's not usually accessible to the user at all. The radio is limited to a fixed number of frequency bands. Between the DAC output of the baseband and the actual radio will be some sort of modulator, which further restricts the kind of signals you can transmit to those using the particular modulations required for that radio.

You might be able to find a way of putting it into "emissions test mode", which causes it to emit a full-power non-modulated signal on a particular band. You're only supposed to do this in sealed RF test chambers.