fix(split): Allocate array of peripheral indexes for BAS proxying.

[petejohanson]

Properly pass address of static peripheral index array to the BT_GATT_CHARACTERISTIC to ensure we read the correct battery level when requested.

Draft for now, just a guess for a nagging issues that's caused issues for split reliability. Paging @caksoylar for testing.

PR check-list

• Branch has a clean commit history
• Additional tests are included, if changing behaviors/core code that is testable.
• Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• Pre-commit used to check formatting of files, commit messages, etc.
• Includes any necessary documentation changes.