Improve untrusted sources UX for app side-loading

When an app is side-loaded we request the user to
enable installs from untrusted sources by sending
them to setting. After the user enables untrusted
sources they have to start the install again
instead of continuing from where they were interrupted
by the settings UI.

With this change the settings activity is started
for a result by the package installer, so if the
settings app wants to support non-interruptive side-
loading flow it can check whether the calling package
is the instaler and if true finish itself returning
RESULT_OK.

bug:26510156

Change-Id: I32d63b7535a84fa2b5b8837cff7d09f6de4a3475