Revert "rdbms: codify DatabaseDomain table "_" prefix convention"

This partially reverts commit c4e284f1130cdc2a4811120594bdb8013d17eaaa
(adding _ to prefixes in tests and readbility / code style fixes are preserved).
It broke all installations which used a table prefix not ending in underscore.

Bug: T227147
Change-Id: I99fc0a2f0fc7b5b321d75fd885763688f92f6d04
(cherry picked from commit 2dad39b5a778c1d472acff8e59188f22d73a8621)