Add the init_boot partition sepolicy

Tagging the partition as a boot_block_device so everything that had
permission to read/write to the boot partition now also has permissions
for this new init_boot partition.

This is required for update_engine to be able to write to init_boot on
builds that are enforcing sepolicy.

Bug: 222052598
Test: adb shell setenforce 1 && update_device.py ota.zip

Merged-In: Ic991fa314c8a6fdb848199a626852a68a57d1df5
Change-Id: Ic991fa314c8a6fdb848199a626852a68a57d1df5