Disable DSU in recovery mode

DSU (modifying mounting paths) will cause OTA update
to fail in recovery mode

Bug: 315887685
Test: 1. enter DSU mode
      2. adb reboot recovery
      3. select "Apply update from ADB"
      4. adb sideload some-ota-update.zip
(cherry picked from https://android-review.googlesource.com/q/commit:222ffc59197a80b3a92e4755e14a570243b8bcbc)
Merged-In: I6aec86893b7f8aa9e34f158269ebe2fd9dd98b33
Change-Id: I6aec86893b7f8aa9e34f158269ebe2fd9dd98b33