Try disabling phenotype flag syncing earlier.

Before this change, we disable phenotype flag syncing during the test,
but it seems to be too late. The test were failing because the device
got flags from the server before syncing was disabled.

This change tries to disable phenotype flag syncing by a test preparer
to make it happen earlier.

Bug: 287005577
Test: Presubmit
Change-Id: Iffb1ff8e7654452c9ef62ba224177c6bcbee45ed
2 files changed