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 vatFAT 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.