Cleanup work on DecryptionFailureTracker (#12546)

* Inline `DecryptionFailureTracker.addDecryptionFailure`

* Remove redundant TRACK_INTERVAL

There really doesn't seem to be much point to this batching up of decryption
failure reports. We still call the analytics callback the same number of times.

* Rename `trackedEvents` to `reportedEvents`

* Fix incorrect documentation on `visibleEvents`

This *does* overlap with `failures`.

* Combine `addFailure` and `reportFailure`

* Calculate client properties before starting reporting

src/components/structures/MatrixChat.tsx

@@ -1591,7 +1591,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
         // tracked events across sessions.
         // dft.loadTrackedEventHashMap();
 
-        dft.start(cli);
+        dft.start(cli).catch((e) => logger.error("Unable to start DecryptionFailureTracker", e));
 
         cli.on(ClientEvent.Room, (room) => {
             if (cli.isCryptoEnabled()) {