Change text for data warning and data limit.

This change modifies the "data warning" text to show either nothing, only
warning, only limit, or warning and limit, based on which of the values are
set. It also remove the capitalization of the first letter of the texts.

Bug: 70950124

Test: manual
Test: make RunSettingsRoboTests
ROBOTEST_FILTER=DataUsageSummaryPreferenceControllerTest
Change-Id: Ifebb411b906719c6e2f343f48d825f7b6f17af53