Make MSC2858 implementation match the unstable policy of the spec

See https://github.com/matrix-org/matrix-doc/pull/2858/files#r543567196

matrix-sdk-android/src/main/java/org/matrix/android/sdk/api/auth/Constants.kt

@@ -33,5 +33,6 @@ const val REGISTER_FALLBACK_PATH = "/_matrix/static/client/register/"
  * Ref: https://matrix.org/docs/spec/client_server/latest#sso-client-login
  */
 const val SSO_REDIRECT_PATH = "/_matrix/client/r0/login/sso/redirect"
+const val MSC2858_SSO_REDIRECT_PATH = "/_matrix/client/unstable/org.matrix.msc2858/login/sso/redirect"
 
 const val SSO_REDIRECT_URL_PARAM = "redirectUrl"

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/auth/data/LoginFlowResponse.kt

@@ -42,6 +42,6 @@ internal data class LoginFlow(
          * the client can show a button for each of the supported providers
          * See MSC #2858
          */
-        @Json(name = "identity_providers")
+        @Json(name = "org.matrix.msc2858.identity_providers")
         val ssoIdentityProvider: List<SsoIdentityProvider>?
 )

vector/src/main/java/im/vector/app/features/login/LoginViewState.kt

@@ -72,9 +72,11 @@ data class LoginViewState(
     fun getSsoUrl(providerId: String?): String {
         return buildString {
             append(homeServerUrl?.trim { it == '/' })
-            append(SSO_REDIRECT_PATH)
             if (providerId != null) {
+                append(MSC2858_SSO_REDIRECT_PATH)
                 append("/$providerId")
+            } else {
+                append(SSO_REDIRECT_PATH)
             }
             // Set a redirect url we will intercept later
             appendParamToUrl(SSO_REDIRECT_URL_PARAM, VECTOR_REDIRECT_URL)