Print OTA merge progress state in logs

Currently OTA merge process prints enum values in logs in numerical
form. This patch changes the log messages from numerical enums to
more friendly names.

Test: Manual OTA

===========Logs==========
CheckTargetMergeState for vendor_dlkm_b returned: Merging
ProcessUpdateState handling state: Merging
CheckTargetMergeState for product_b returned: Merging
CheckTargetMergeState for vendor_dlkm_b returned: MergeCompleted
ProcessUpdateState handling state: Merging
CheckTargetMergeState for product_b returned: MergeCompleted
ProcessUpdateState handling state: MergeCompleted

Bug: 234518211
Change-Id: If690d3ea86e6fe37e8b3e4bd52e87b70ae92495c