services/shell: Turn on sandboxing for mojo apps.

Turn on sandbox for mojo apps by default. Apps that need to run
without a sandbox need to include 'shell:unsandboxed' in their
capabilities in manifest for mojo:shell.

Update manifests for mojo:ui, mojo:font_service, mojo:file_system,
and mojo:leveldb services to include 'shell:unsandboxed'.

BUG=645602, 646376

Cr-Commit-Position: refs/heads/master@{#419891}
7 files changed