Adding app specific screen for "External Sources"

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