storageproxy: Support POST_COMMIT sync for all commands

Previously we did not support STORAGE_MSG_FLAG_POST_COMMIT for anything
but RPMB operations (in which case it was a no-op). We need to support
this flag in order to store a superblock in non-secure storage, as we
need that write to commit atomically wrt all other writes.

Test: com.android.storage-unittest.nsp
Bug: 228793975
Change-Id: Ia453c1916970e0b65a91e42f18b920ac4e1f01db