The android phone can be used in USB host mode to power an arduino, pen drive, mouse.
Is it possible to use the android phone to power a raspberry pi?
It very much depends on what you're using your RPi for, how much current it's drawing, and how much power the phone supplies.
The simplest and 'correct' answer in terms of what you 'should' do is that no current phone provides sufficient power to meet the Raspberry Pi minimum requirements, so you are always at risk of problems.
In reality, it's certainly possible to power a raspberry Pi from a much lower power, power source. It's not advisable, however, if you require stability and reliability. Eg if you're doing it for anything other than experimentation/education purposes.
Even if a USB OTG Host such as a phone would supply the current needed to run the RPi you'd have to consider the over-all power consumption of the RPi compared to the batteries of your phone. Find values at How much energy does the raspberry pi consume in a day?
Until this answer is posted,No. Yes an Android device can act as a USB host but it won't supply enough power to power up a RPi. If you require a connection between RPi and a host Android device via USB you need some work around for supplying power into RPi
Edit 1 : These answer not applicable to all Android Device,since some of OEM allow their devices to act as Host.