[FEATURE REQUEST] - Vault Auto Lock

[ricassiocosta]

Is your feature request related to a problem? Please describe.
In high-end smartphones with a lot of memory available, the app can potentially remains open and accessible after a long time if the user forget to manually lock the Vault or close the app.

Describe the solution you'd like
A feature to auto lock the vault after X seconds (configurable) or always the app goes to background (also configurable) would be nice.