Separate Android notification data by account

[gnprice]

Currently we maintain the following data structure in our Android-native code that handles notifications:

/**
 * The Zulip messages we're showing as a notification, grouped by conversation.
 *
 * Each key identifies a conversation; see [buildKeyString].
 *
 * Each value is the messages in the conversation, in the order we
 * received them.
 *
 * When we start showing a separate notification for each  [Identity],
 * this type will represent the messages for just one [Identity].
 * See also [ConversationMap].
 */
open class ByConversationMap : LinkedHashMap<String, MutableList<MessageFcmMessage>>()

/**
 * All Zulip messages we're showing in notifications.
 *
 * Currently an alias of [ByConversationMap].  When we start showing
 * a separate notification for each [Identity], this type will become
 * a collection of one [ByConversationMap] per [Identity].
 */
class ConversationMap : ByConversationMap()

As those comments foreshadow, we should separate out this data by which account/identity the user received it for. As is, because we don't, we end up showing conversations in different accounts jumbled together in a single summarized notification.

This will then be a building block for #2691.

We might also first use the separated data structure to start showing separate notifications per account within our current style of notification; that'll be worthwhile just if the work consists mostly of refactors we need anyway for #2691.

[gnprice]

This was accomplished as part of #4842. We actually stopped keeping our own data structure here, leaving it to the OS, and we arranged there for the data to be appropriately separate.