Don't ask for password on app start up if we the user has already created an account

[piyalbasu]

Currently in Freighter, we ask for a user's password at the beginning of a session because all we store in localStorage is the user's encrypted key blobs. The password is needed to get both the public and private keys. The private key is erased after the session times out. The public key is erased when the user is upgraded/reloads the extension

We want to move to a UX where the password is only required when signing a tx (.i.e, when we need the private key). In order to accomplish this we will need to:

Acceptance Criteria:

• Store all the user's public keys in localStorage so they persist after extension upgrade

Companion ticket:

• Create password prompts when the user is about to sign a tx, but the private key is not available (this already exists for send/swap and sign tx/auth entry/message, but not change trustline). This can be handled in Adding an asset should feel like making a transaction #1671

[piyalbasu]

Suggestion from Cassio: Include a "require password on app startup" setting for users that are more conservative