Export flags used in FlaggedApi annotations

Before calling a flagged API, client code must check the value of the flag
which gates it. Those flags must be exported in order to be accessible from
containers other than the container where the flag and the API are hosted.

Bug: 320984775
Bug: 322839671

Test: m all_aconfig_declarations
Test: printflags --format='{fully_qualified_name}:{is_exported}' | grep true
Change-Id: I9140cf907183477d9e9d761242d9735cc2f84947