Do a reboot before starting each test (3/n)

This patch improves test isolation.

Sometimes a previous test fails with reboot in progress. Then the next
test is in an unstable state since reboot could happen at any moment and
fail the device test.

Doing a reboot before each test starts protects us from pending reboot
from the last test and improve isolation.

Bug: 143999213
Test: atest StagedRollbackTest
Change-Id: Idd0fd1cc6810659990b406640efd7404de1dcd07