Follow rename of dex_preopt to dex_preopt.enabled

I54a4b1de697a08be20ab65d2a5dc43ce0046692d renamed dex_preopt to
dex_preopt.enabled to make room for more properties under
dex_preopt.

Test: m checkbuild
Change-Id: If976a116e4cc332dd00784b4179b9043faf86edf
1 file changed