According to [R-Pi Troubleshooting](http://elinux.org/R-Pi_Troubleshooting#Green_LED_blinks_in_a_specific_pattern) 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](https://www.raspberrypi.org/downloads/raspbian/) 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](https://www.raspberrypi.org/documentation/hardware/raspberrypi/bootmodes/bootflow.md).