proposal: Move proposal_t from libcharon to libstrongswan

This allows us to use it without having to initialize libcharon, which
was required for the logging (we probably could have included debug.h
instead of daemon.h to workaround that but this seems more correct).

src/libcharon/Android.mk

@@ -16,7 +16,6 @@ config/backend_manager.c config/backend_manager.h config/backend.h \
 config/child_cfg.c config/child_cfg.h \
 config/ike_cfg.c config/ike_cfg.h \
 config/peer_cfg.c config/peer_cfg.h \
-config/proposal.c config/proposal.h \
 control/controller.c control/controller.h \
 daemon.c daemon.h \
 encoding/generator.c encoding/generator.h \