Flush buffer to filesystem before fsync

If power is lost immediately after updating a config file it is possible
to save an empty file in its place.  By flushing the stream buffer to
the filesystem before flushing the filesystem to the disk this risk is
mitigated;

Bug: 122607444
Test: Update Bluetooth Device name and interrupt power to device.
Change-Id: Iaf89c4d9c594e925507e713a9febbf23a7cdae43