Enable verity to updated priv app if root hash exists

With the flag ro.apk_verity.mode set to non-zero, on install time,
package manager now tries to enable verity for priv apps if the root
hash is included in the APK's Signing Block.

Test: Install a priv app with verity root hash in the Signing Block.
      Saw the tree is built, passed to installd.
      Failed on ioctl at the moment since kernel is not ready. (need the final installd patch)
Bug: 30972906

Change-Id: I17b6589b44485fb377d5618da55fb2a4572d4ae8