[keyring-controller] Remove #getMemState returns

[mikesposito]

Explanation

This PR is an intermediate refactor needed for #4192.

Since operations are eventually rolled back, the internal method will not be able to return the last controller state, risking returning a stale one. Moreover, function returns make more sense now.

This PR needs these changes to be merged first:

• [keyring-controller] Lock controller mutex on write operations #4182

References

@metamask/keyring-controller

• BREAKING: Change various KeyringController methods so they no longer return the controller state
  • Changed addNewAccount return type to Promise<string>
  • Changed addNewAccountWithoutUpdate return type to Promise<string>
  • Changed createNewVaultAndKeychain return type to Promise<void>
  • Changed createNewVaultAndRestore return type to Promise<void>
  • Changed importAccountWithStrategy return type to Promise<string>
  • Changed removeAccount return type to Promise<void>
  • Changed setLocked return type to Promise<void>
  • Changed submitEncryptionKey return type to Promise<void>
  • Changed submitPassword return type to Promise<void>

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate

[mcmire]

I suspect this PR may need to be rebased, but regardless, this PR makes sense.

[mcmire]

One nit, but looks good regardless (we can always tweak the changelog later).