Guide user towards adoption when card is "empty".

When a newly inserted SD card is empty, we'd like to guide the user
towards adopting it.  Similarly, if the card contains personal media
like photos, we'd like to guide the user towards using it as portable
storage.

Do this by quickly hunting around on the card for files under various
well-known directories.  Special logic to ignore bundled "helper"
apps included from the SD card factory.

Test: bit FrameworksCoreTests:android.os.EnvironmentTest
Bug: 69128181
Change-Id: I10e43d2e76379fac5137eb3810742c33f5f57d80