Enforced app op permission in package installer

Apps targetting O and above need to declare REQUEST_INSTALL_PACKAGES
permission in order to start package installer. Only apps exempt are
downloads provider and document manager apps. And only these apps can
use EXTRA_ORIGINATING_UID to blame other uids for app installs. Also
added a special wraning dialog for when system cannot identify the
source of the package.

Test: gts-tradefed run gts -m ExternalSourcesNegative

Bug: 35916776,
     33351244
Change-Id: I74b242355807502c4e1bf0e7dbdb9845dc725e48