Bring up the erase animation early for data wipe

Right now the "Erasing" animation displays after the merge step
during the FDR process; and the merge can take 3-4 minutes. The
users maybe confused about the blank screen and forcefully
reboot the device.

Bug: 181636823
Test: add sleep merge, check the animation displays correctly.
Change-Id: Ib23b1ed3a84e95640271a429c51a3d3c142dc404