Adding Manage External Sources Settings

Added a settings fragment to manage external sources. It lists all
applications that have either requested REQUEST_INSTALL_PACKAGES or have
their app op changed from default.

Test: Will include in follow-up CL. Tracked in b/33792674

Bug: 31002700
Change-Id: Ibd2a1922be214b62aec4eefa45f7b9691256b205
8 files changed