releasetools: Perform erase commands first.

We used to erase all the unused blocks at the end of an update. In order
to work around the eMMC issue in b/28347095, we move part of the erase
commands, which won't be used during the update, to the beginning. The
reset would be erased at the end. This is in hopes of avoiding eMMC
getting starved for clean blocks which would otherwise fail an OTA.

Bug: 28347095
Change-Id: Ia96b06216b35d6700858687a66734af36d0ec213