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: If38349f349781541673f93638e536a4f2a184bf6