Fix the permission of stashed blocks created by updater

Our updater created the stashes with root permission. This causes an
access denial when the RecoverySystem service tries to clean up these
blocks after a failing update. As a result, the subsequent OTA updates
may fail due to insufficient cache space.

Bug: 36457133
Test: stashed blocks cleaned successfully after reboot
Change-Id: If0ca99638cdfa1033646f29d9cc92b5ff1bacac1