build: drop the nonFree flavor, Sentry and Play Services

The fork targets GrapheneOS and ships only FOSS builds. The nonFree
flavor added Sentry crash reporting, wired to upstream's Sentry project,
and Play Services SMS one-time-code autofill, which needs Google's
servers. Neither is wanted, so remove the flavor dimension entirely:

- Delete app/src/nonFree, the SentryPlugin, the sentry-stub module, the
  Sentry and Play Services catalog entries, and the Sentry scope setup
  in Application.
- Move the free AutofillSmsActivity stub into main; it keeps the SMS
  OTP feature off exactly as the free build did. A ponytail note marks
  the FillOtpFromSms paths for later removal.
- SlimTests no longer needs to skip a nonFree variant.
- Workflows and docs use :app:assembleDebug / :app:assembleRelease. A
  bare assembleRelease would also verify library modules in release
  mode, where ui:compose fails on a pre-existing AppCompat attribute
  reference the old flavored task never touched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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