Unset INSTALL_DISABLE_VERIFICATION flag if install is not from system

Bug: 138650665
Test: pushed manual privapp that attempts an install
  with INSTALL_DISABLE_VERIFICATION flag.
  Checked that PackageVerifier was called.
Test: atest CtsSecurityTestCases:android.security.cts.PackageInstallerTest
Test: atest CtsStagedInstallHostTestCases
Change-Id: I041d2e5f6d2609bfe9475f24eefb936f0c8e2a32
(cherry picked from commit dfd3641ec9fa6a428af62540e53ac22e6ba0b412)