fix A/B partition flags to match ABL

[andiradulescu]

I noticed this on Void, since abctl was using the incorrect bits and after a number of reboots I was getting "Failed to boot". On serial:

Slot _a is unbootable, trying alternate slot
Err: line:1522 FindBootableSlot() status: Load Error
Err: line:1060 LoadImageAndAuth() status: Load E

The problem is that setActiveSlot() and getActiveSlot() wrote and read A/B partition flags at wrong bit positions, 6 bits too high. This rewrites both to match ABL's SetActiveSlot().

PR #92 misinterpreted edl's AB_FLAG_OFFSET = 6 (a byte offset into the 8-byte attributes field) as a bit offset added to 48, making AB_FLAG_OFFSET = 54 instead of 48. All flag writes landed at bits 56+ which ABL ignores.

This appeared to work because after the precedence fix (6ff8067), bits 48-55 from GPT images were preserved untouched, and ABL booted from those pre-set flags. But with retry=7, successful=0 from the GPT image, the device relies on mark_boot_successful — if that also writes to the wrong bit, the device bricks after exactly 7 reboots.

Supersedes #118 which fixes precedence but not the offset.

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Full research:
ab-flag-comparison.md
ab-flag-readings.md