fastbootd: Fix partition size testing issues.

Partition sizes must be reported with an "0x" prefix for fuzzy_fastboot.
Also, with dynamic partitions, the size of a partition can be 0.

Bug: 78793464
Test: fuzzy_fastboot Conformance.PartitionInfo does not error on
      partition sizes when using fastbootd

Change-Id: I4148440bd9ed420878940829618cbf8cee85bf6a