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
(cherry picked from commit dbdeb8da5ab0dad02f25edfe313e59d506e718f3)