remount: On initial overlay setup, return 0 instead of MUST_REBOOT

aosp/I212bdb0e97016dec50618962d7c24f46d35764c7 changes
`remount` to return MUST_REBOOT after initial overlay setup.
This causes DsuGsiIntegrationTest to fail because the non-zero exit code
cause the test script to think the `remount` command failed.

Change it so that we return 0 instead, to indicate that we
"successfully" setup a new overlay.
We should only return non-zero on unrecoverable error, like when we
failed to disable verity, failed to setup overlay, failed to perform
MS_REMOUNT.

Bug: 241179247
Test: DsuGsiIntegrationTest
Change-Id: I280ffa988118c59e366cdd5bd1479bb43896c278