Whn i give it power through PC via the power cable,the sensor lights up n work perfectly.
What power cable are you talking about? The only cable between the PC and the Arduino is usually the USB cable, which does supply power but also allows the exchange of data.
But when i plug in the usb cable to the usb port of the arduino to my android device
I know what a USB cable is, and that the Arduino has a USB port. I'm having a hard time figuring out where you are putting the other end of the cable, though.
Arduino shud be able to supply pwer to breadboard n android device simultaneously.isnt it?
The Arduino is not a power supply. Where did you get that idea? It consumes power. Your phone might be supplying power to the Arduino, or might not. But, the Arduino is not powering the phone.
Here is the circuit diagram..Blue block is arduino mega adk
That is not a circuit diagram. Post a picture where the wiring can be clearly seen.