Reenable presubmit runs for BackupSettingsTest now that it's fixed

Bug: 37684646
Test: mmma -j40 frameworks/base/core/tests/coretests, then
adb install -r ${ANDROID_PRODUCT_OUT}/data/app/FrameworksCoreTests/FrameworksCoreTests.apk, then
adb shell am instrument -w -e class android.provider.SettingsBackupTest com.android.frameworks.coretests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I1927c08e445b914b84b55d40aba68c234d6abcb5