[lldb][nfc] Move broadcaster class strings away from ConstString (#89690)

These are hardcoded strings that are already present in the data section
of the binary, no need to immediately place them in the ConstString
StringPools. Lots of code still calls `GetBroadcasterClass` and places
the return value into a ConstString. Changing that would be a good
follow-up.

Additionally, calls to these functions are still wrapped in ConstStrings
at the SBAPI layer. This is because we must guarantee the lifetime of
all strings handed out publicly.