Otapreopt: Fix property handling

Follow-up to commit 1b9d9a60.

In the previous CL, a strncpy was translated incorrectly to a
strlcpy. Fix the glitch.

Bug: 36655947
Bug: 66996038
Test: manual A/B OTA
Change-Id: I387ff8436de351a3cec5458b3ef013e15905a046