ANDROID: freezer: export the freezer_cgrp_subsys for GKI purpose.

Exporting the symbol freezer_cgrp_subsys, in that vendor module can
add can_attach & cancel_attach member function. It is vendor-specific
tuning.

Bug: 339371220
Bug: 182496370
Bug: 281920779

Signed-off-by: Zhuguangqing <zhuguangqing@xiaomi.com>
Change-Id: I153682b9d1015eed3f048b45ea6495ebb8f3c261
(cherry picked from commit ee3f4d28)
(cherry picked from commit 8a90e4d4)