Add feature flag for new DataUsageSummary fragment.

Creates a new fragment DataUsageSummaryLegacy, which is used
instead of DataUsageSummary when the feature flag is off (default
behavior).

Copies the current DataUsageSummary.java, data_usage.xml, and
DataUsageSummaryActivity to create the legacy versions. This
change allows for uninterupted development of new features in the
DataUsageSummary fragment.

Test: manual
Test: make RunSettingsRoboTests
Bug: 70950124
Change-Id: I9d79e108bbfc1c8ec620f86c0183f4ed3f41c9d7