Adding app specific screen so that third party apps and package
installer can directly intent there to direct users to change the
setting. Also added automater tests.
Test: adb shell am instrument -w -e class \
'com.android.settings.applications.ExternalSourcesSettingsTest' \
'com.android.settings.tests/android.support.test.runner.AndroidJUnitRunner'
Bug: 33792674
Bug: 35487166
Change-Id: I5a481ee7032979df94a9e267ea74aa90da6a0906
f59d2115ed