Fix PIA enrollment URLs in deep links and container sync

[frankii91]

Problem

When the push_use_pia_scheme enrollment policy is enabled, privacyIDEA
returns Push enrollment URLs with the pia://pipush/... scheme.

Two supported enrollment paths could mishandle these URLs:

1. Smartphone-container synchronization passed every server-provided
   enrollment URL directly to OtpAuthProcessor, which only accepts
   otpauth://. Synchronization could therefore complete while silently
   omitting the Push token, leaving the server-side token in clientwait.
2. On Android, external enrollment links were claimed by Flutter's built-in
   deep-link handler even though the app uses the app_links plugin. Flutter
   interpreted the enrollment URI as a Navigator route and raised a
   Could not find a generator for route error instead of delivering it to
   the existing token-import flow.

QR enrollment already routed pia:// URLs through PiaSchemeProcessor.

Change

• Route enrollment URLs returned by container synchronization through
  TokenImportSchemeProcessor.processUriByAny. This selects
  PiaSchemeProcessor for pia:// and preserves the existing
  OtpAuthProcessor path for otpauth://.
• Opt out of Flutter's built-in Android deep-link handler so the existing
  app_links integration receives external enrollment links. This follows
  Flutter's documented configuration for applications using a third-party
  deep-link plugin.

No URL schemes or Android intent filters are changed.

Test coverage

The container synchronization regression test builds an encrypted response
containing a pia://pipush/... enrollment URL and verifies that it produces a
container-associated PushToken with the expected rollout URL and enrollment
credential.

Validation after both changes:

• deep-link notifier and container API focused suites: 17 tests pass;
• complete Flutter test suite passes;
• Android manifest XML validation passes;
• git diff --check passes.

flutter analyze --no-pub reports only the existing upstream diagnostics:
57 info messages and one pre-existing warning in token_folder.dart. The
changed files introduce no new warning or error.