aconfig: cpp codegen update: Move server_configurable_flags header include

Move server_configurable_flags header include away from exported header,
instead put it in the flag provider headers. Otherwise, it can cause
compilation error if the source code who wants to use the generated c flag lib has
not added dependency on server_configurable_flags yet.

Bug: b/279483801
Test: atest aconfig.test
Change-Id: Ib75877ee88f995caf72b3fd2554c3da195032c14