Renaming intent action for unknown sources

Renaming action MANAGE_EXTERNAL_SOURCES to MANAGE_UNKNOWN_APP_SOURCES to
be clearer about its purpose

Test: adb shell am instrument -w -e class \
'com.android.settings.applications.ExternalSourcesSettingsTest' \
'com.android.settings.tests/android.support.test.runner.AndroidJUnitRunner'

cts-tradefed run singleCommand cts -m ExternalSources

Bug: 35812498
Change-Id: Id3601c6b908d045c008142b9ee214c8dc3ac23ef