Checking for app op OP_INSTALL_PACKAGES in PackageInstaller

PackageInstaller now ignores the secure settings INSTALL_NON_MARKET_APPS
and uses the app op INSTALL_PACKAGES to decide whether it should allow
installation of an app coming from an external source. This allows the
user to control with more granularity which app sources they trust.

Test: Tracking tests in b/33792674

Bug: 31002700
Change-Id: Ifa2961b1e2b4cf622ab52a2d545048ba065fa0d4