Don't destroy the FalsingManager in Wallet.

When FalsingManager#cleanupInternal is called, it no longer produces
valid results. With this change, we check that the FalsingManager
is not used after being destroyed, and also avoid destroying it in
WalletScreenController.

Fixes: 188174214
Test: manual
Change-Id: I0ce67de5a326b56dee11c1d63c1d592640c0713d