Harden CreateMessageActivity against external navigation injection

[DenBond7]

This change hardens the exported compose entry point against untrusted external intents reaching internal send-only flows.

Problem

CreateMessageActivity is intentionally exported to support normal Android share and mail intents. However, untrusted external intents were able to influence internal navigation behavior more than intended and reach the outgoing-message creation path directly. That flow could then process attacker-controlled attachment URIs using FlowCrypt’s own app identity.

What changed

• sanitize incoming Intent extras before Navigation can treat them as internal deep-link state;
• preserve normal public compose/share entry behavior while preventing external navigation injection;
• Keep outgoing attachment handling restricted to FlowCrypt-managed sources only.

Why

External intents should only initialize the normal compose experience. They must not be able to route directly into internal send logic or supply unsafe attachment locations that are later opened with app privileges.

What verified:

• normal ACTION_SEND, ACTION_SEND_MULTIPLE, and mailto: flows still open compose as expected;
• internal reply/forward/draft flows still work;
• crafted external intents can no longer reach the internal outgoing-message creation dialog;
• unsafe attachment URIs outside FlowCrypt-managed locations are rejected.

close #3227

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)
• Difficult to test (explain why)
• Not worth testing
• Tests will be added later (issue #...)
• Tests added or updated

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities