fs_mgr: Add getter for androidboot.boot_part_uuid

In order to make booting from some media types (like USB) more robust,
the bootloader will be extended to support passing the partition UUID
that it loaded the kernel from. It can pass this via kernel
commandline or via bootconfig. Add a way to get this.

Bug: 316324155
Test: Use the getter in a future change

Change-Id: Iab04742c0f2666db18dc48bcaaa2869eba405748