TableLogBuffer: Default columnPrefix to "".

Most uses of TableLogBuffer are setting columnPrefix to "", so we should
make that the default and save most people from writing it explicitly.

This also removes the `columnPrefix = ""` in all the status-bar-related
code. Future CL(s) will remove it from other places.

Bug: N/A
Flag: EXEMPT refactor
Test: Dump TableLogBuffers with and without column prefixes -> verify
they look the same

Change-Id: I8fbe2b7dbf35f45ece10a23d5c20da3e5d7b7772