Previous code assumed the active account could always be determined from
the account manager.
This causes a few problems.
1. The account manager has to handle the case where there is no active
account (e.g., the user is logging out of the last account). This meant
the `activeAccount` property had to be nullable, so every consumer of
that property either used it with a `let` or `!!` expression.
2. The active account can change over the life of the UI component, for
example, when the user is switching accounts. There's a theoretical race
condition where the UI component has started an operation for one
account, then the account changes and the network authentication code
uses the new account.
3. All operations assume they operate on whatever the active account is,
making it difficult to provide any features that allow the user to
temporarily operate as another account ("Boost as...", etc).
This "ambient account" was effectively global mutable state, with all
the problems that can cause.
Start to fix this. The changes in this commit do not fix the problem
completely, but they are some progress.
Each activity (except LoginActivity) is expected to be launched with an
intent that includes the ID of the Pachli account it defaults to
operating with. This is `pachliAccountId`, and is the *database ID*
(not the server ID) of the account. This is non-null, which removes one
class of bugs.
This account is passed to each fragment and any piece of code that has
to perform an operation on behalf of this account. It's not used in
most of those places yet, that will be done over a number of followup
PRs as part of modernising each module.
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