Expose GetRamdiskFormat and use in ota_utils.py

Some OTA files may have additional OTA props; when this is being computed within
ota_utils.py -> ComputeRuntimeBuildInfos, the ramdisk type needs to be passed into
PartitionBuildProps.FromInputFile as not all ramdisk may still use gzip. This check
is already defined in common.py, so this change also exposes the function GetRamdiskFormat.

Bug: 231075507

Signed-off-by: TJ Rhoades <tjr@microsoft.com>
Change-Id: If1b93b887990f0d90df2c6003122821c1e66a1ac