Emulator helper so tests start in seconds, not minutes

The Google TV emulator was launched by hand with -no-snapshot, which forces
a cold boot every time. tests/helpers/emulator.sh boots from the AVD's
default_boot snapshot (~7s) and refreshes that snapshot on stop, so the next
run stays fast. It also wraps the two things a test run needs afterwards:
installing an APK and pulling a screenshot into tests/runs/.

CONTEXT.md's example dialogue still described Xtream credentials entered
through the Remote, which ADR-0005 and ADR-0007 replaced with device flow and
phone-side onboarding.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CONTEXT.md

@@ -73,9 +73,10 @@ Channel, no full guide timeline.
 ## Example dialogue
 
 > **Dev:** "Does the **Remote** need Backend credentials?"
-> **Domain expert:** "No — the user enters the Xtream credentials of a
-> **Quelle** once via the Remote, the **TV-App** stores them and is the only
-> one talking to the **Backend**."
+> **Domain expert:** "No — the Remote is where **Onboarding** happens, not
+> where credentials live: the user types the server URL there and finishes the
+> IdP login on the phone. The **TV-App** holds the tokens and is the only one
+> talking to the **Backend**. A generic M3U **Quelle** needs no login at all."
 
 ## Flagged ambiguities