A new ui flow for screen pinning feature

The purpose of this change is to resolve a number of security issue
around screen pinning.
See more detail go/screen-pinning-allows

There're a few change for Settings app.

- Rename the screen pinning to app pinning.
- Change the string description for app pinning.
- Pop up a warning dialog while user is enabling this feature.

Test: Rebuilt rom and see new ui flow.
Bug: 151332926
Change-Id: Ife07d7b95ab5dccb2aed7f2bc8fa32f97763bd63