Special handling of priv-apps in Zygote.

If pm.dexopt.priv-apps is set to false, disable verifier and
only allow loading oat files from system.

bug: 30972906
bug: 63920015

Test: works as expected when pm.dexopt.priv-apps is true or false
Change-Id: Ib9e80c9b7b4106e82c0b9d1c7fbb8065c190ac1f