Add feature context

Currently only used by app-ops, but can be used by other modules too.

AppOps will collect app-ops based on feature. Only the most basic
parts of app-ops are supporting features yet. The rest will be added
later.

Test: atest CtsAppOpsTestCases
      atest RunSettingsLibRoboTests
      atest com.android.server.wm.ActivityStarterTests
      m -j doc-comment-check-docs
Bug: 136595429
Change-Id: Ia664f4ff1c1c2ceea721d76dc491ec50c237e9ce