Disable dexpreopting when TARGET_BUILD_APPS is used.

This moves the condition from Soong to make, so that
config.UnbundledBuild() in Soong no longer implies disabling
dexpreopting.

Test: TH, in particular builds green on ub-launcher3-master
Bug: 157549171
Change-Id: I7273be8591c9d627d89e033b641139dacc275806