Move WITH_DEXPREOPT default value

This allows setting the variable READONLY after reading the BoardConfig.

This is safe to do because this flag is only set in BoardConfig.mk
files. The same is not true for many of the other default preopt flags,
like DEX_PREOPT_DEFAULT or WITH_DEXPREOPT_BOOT_IMG_AND_SYSTEM_SERVER_ONLY
which are set in mixture of product / board config files.

Test: boot blueline
Change-Id: I961ba581f9790bcdc7cf6b36d8b53bcdb8861a92