Update instructions on how to build and run briar-headless

Closes #2370

See merge request briar/briar!1726

Remove jcenter repository from gradle files

See merge request briar/briar!1736

Merge redundant strings

See merge request briar/briar!1737

Add hints that both users need to scan each other's QR codes/add each other's links

See merge request briar/briar!1734

Prevent crash in TestDataCreator if blogs or forums are not enabled in core

Closes briar-desktop#420

See merge request briar/briar!1735

Illustration revamp

See merge request !1722

Send ClientVersionUpdatedEvent for each client state change, not only on version updates

See merge request briar/briar!1724

in order to avoid two splash screens from being shown.

This seems to be necessary. Without the location turned on, the hotspot does not start showing a p2p error.

We need to have those permissions before doing things like accessing the Bluetooth address. So we force-disable the Bluetooth plugin if the permission is not granted. The UI then forces the permission before allowing to enable the plugin.

For some reason SettingsActivity does not need to be exported for it to be launched from system app settings.

androidx.test.ext:junit needed to be upgraded because it somehow brought in an activity without exported attribute

Speed up BridgeTest by only retrying bridges that have failed

See merge request briar/briar!1732

Update bridges

See merge request !1731