libavb_atx: Remove GSK rollback index and increase key size.

- The GSK rollback index was redundant with existing vbmeta rollback
protection. It has been removed.
- ATX now uses 4096-bit keys.
- Added a test data generator script.

This change breaks compatibility of ATX data structures.

Bug: 35767054
Test: unit tests
Change-Id: Ifff377c88abd531a394afe9e71fbc1307a7f834e
15 files changed