Change the default value of sync mode to NORMAL

When journal mode is WAL ,the default sync mode is FULL.
In Android S when journal mode is WAL,the default sync mode is NORMAL.

In the annotation of google it's said when journal mode is WAL,
the best sync mode should be NORMAL.

Incorrect sync mode cause performance drop.

Bug: 246753316
Test: run fs benchmark

Change-Id: I8110d66091cd443db9b09f6e8564f9d97f5e7297