Bump briar core to current master and define okhttp_version

briar @ 20b52804

-Subproject commit 166433dcc8f9aeb6bd05ef51a575eb2881a1730f
+Subproject commit 20b52804bf4a4306739eeafa4de9f312ffdea9b9

build.gradle.kts

@@ -21,6 +21,7 @@ buildscript {
     // keep version here in sync when updating briar
     extra.apply {
         set("dagger_version", "2.33")
+        set("okhttp_version", "3.12.13")
         set("tor_version", "0.3.5.17")
         set("obfs4proxy_version", "0.0.12-dev-40245c4a")
         set("junit_version", "4.13.2")