storageproxyd: Sync parent dir when creating a file

Switches to syncing the parent directory immediately when creating a new
file rather than lazily waiting for a sync request. Because we only
create a new file when the userdata partition is cleared, this operation
doesn't need to be fast in the normal case. This avoids needing to track
the parent directory for each file for lazy syncing later, since storage
backing files may be in a subdirectory of the root.

Test: m storageproxyd, boot using new service binary
Bug: None
Change-Id: Ibcafae7da493864515c099bd81d48c95b0e5d8c3