Arm backend: Merge decompose/convert meandim pass (#10844)

This change has multiple benifits:
- Cleaner arm_pass_manager
- Use more efficient avgpool2d decompostion in more cases
- Fixes a bug decomposing to avgpool for rank != 4

Note that symmetric_io_quantization is required for the unittests
resulting in only a avgpool op becuase of the way avgpool is annotated.

Signed-off-by: Adrian Lundell <adrian.lundell@arm.com>