Store account data in the same way as room data
Previously, we treated the `MatrixEvents` that were in `this.accountData` in
`MatrixInMemoryStore` as the ground truth and saved those to disk and restored
them back upon load. This did not consider that there are **no emitted events**
when they are restored. Riot-Web was listening for a specific account data
event in order to dynamically update the theme. When the page was reloaded, we
dutifully put the right event in `MatrixInMemoryStore`, but failed to emit an
event to tell Riot-Web this. This led to vector-im/riot-web#3247
This patch fixes it by treating the `/sync` response as the ground truth and
ignoring `this.accountData` entirely. This means that upon load, we will be
injecting an `account_data` key into the initial `/sync` response. This will
cause it to be added to `this.accountData` in the store AND cause the event to
be emitted.
