I have a Raspberry Pi 3 b. I tried to boot it to android things OS. But when I connect the Pi to power, it is showing nothing except a rainbow colour screen and the green led is blinking 7 times. Please help me correct this issue?
According to R-Pi Troubleshooting with 7 flashes of the green led the
kernel.img is not found. I do not have any experience with android things but in general the Raspberry Pi boot loader expects its boot files including the kernel image on the first partition of the SD Card. This partition must be formated with the vat file system. So please check if this conditions are met. To compare, the boot partition of the official supported Raspbian Stretch Lite 2019-04-08 contains this files:
rpi ~$ ls /boot bcm2708-rpi-0-w.dtb bcm2710-rpi-3-b.dtb config.txt fixup_db.dat kernel.img start_db.elf bcm2708-rpi-b.dtb bcm2710-rpi-3-b-plus.dtb COPYING.linux fixup_x.dat LICENCE.broadcom start.elf bcm2708-rpi-b-plus.dtb bcm2710-rpi-cm3.dtb custom-configuration.txt initrd.img-4.14.98-v7+ LICENSE.oracle start_x.elf bcm2708-rpi-cm.dtb bootcode.bin fixup_cd.dat issue.txt overlays bcm2709-rpi-2-b.dtb cmdline.txt fixup.dat kernel7.img start_cd.elf
Further information about the boot process you can find at Boot flow.