Re-submit "Add trailing slash to mainline path claims."

With a workaround for preopt files that get installed into an unknown
path. The BoardConfig defines whether the files go in /system_other,
so allow that.

This reverts commit 46bf4b51.

Test: lunch mainline_arm64-userdebug; m nothing
Change-Id: Ia75670eb6e3730ad0ff17f5b81954b69f6cab22b