ANDROID: Incremental fs: Fix create_file performance

incfs only syncs at createfile time. This was making createfile take
a very long time. It also appears to offer little actual value -
whether we flush or no, if the device crashes the header will be
partial.

Bug: 15356649
Test: incfs_test passes, createfile takes less than half the time
Change-Id: I8f1fa138226868ebfb4a6a41254444af453070c8
Signed-off-by: Paul Lawrence <paullawrence@google.com>