Partially revert the usage of string enum.

There's a regression in showmap_rss_bytes, after looking into the whole
showmap, the increase mostly came from framework, after partially revert
the usage of string enum, the showmap_rss_bytes number seems to decrease
consistently.

Bug: b/289956225
Bug: b/283858001
Test: forrest test
Change-Id: I3f548ac2bc437394d7da7d0a88090dd3aa27d7f8