Add a system API UpdateEngine to switch the slot back and forth

In some cases, the OTA update want to flip the slot to source
without cancelling the entire update. The functionality was
achieved by ResetStatus(). However ResetStatus() will clean up
the update progress and free up space to support virtual A/B.

Details in http://go/no-virtual-ab-switch-off

Test: build
Bug: 187321613
Change-Id: I2abcf8289f5fd573d2b55fa7004ab4dcd175252f