KeyMint HAL: clarify obsolete tags

The tag enum names can't be removed due to AIDL back-compatibility
requirements, and also it's useful to have the values present to avoid
inadvertent reuse.

Update the tag comment text to indicate that these tags are obsolete.

Bug: 191738660
Test: TreeHugger, comment change only
Change-Id: Icbd4c9cd0313f93bc491b49eb9077766d0f44e34