Add a unit test for the AppOps permission for displaying UI overlays.

The test logic is the same as ExetrnalSourcesSettingsTest but I moved/renamed some methods so both AppOps tests can use them.

Test: make -j32 SettingsUnitTests && adb install -r out/target/product/marlin/data/app/SettingsUnitTests/SettingsUnitTests.apk && adb shell am instrument -w -e class com.android.settings.applications.ExternalSourcesSettingsTest com.android.settings.tests.unit/android.support.test.runner.AndroidJUnitRunner
Test: same as above, but for DrawOverlaySettingsTest
Change-Id: Ifc0f0985a4cb4a82a00839aeb4cd4216c777250e