Remove hardcoded itemTypes from SearchTarget

- Introduces componentName and userHandle members to SearchTarget
- SearchTargetEvent now has searchTarget member
- Builder pattern for SearchTarget and SearchTargetEvent
- Search backend should add headers manually instead of launcher inferring sections

Bug: 171026321
Test: Manual
Change-Id: I28e0455e82b925277a17703b9aa061c8f9f15262