android: 2.13.15 - exponential direct-dst cooldown, double-start guard, flap-aware auto-connect

Log-driven fixes from the 10-04 diag/charon analysis:
- direct_tcp: negative-cache window now escalates 45s x2 up to 5 min
  (Telegram was opening ~3000 connects/h to each of two dead hosts,
  waking the radio for every leaked 45s probe); skip the unconditional
  fresh-socket retry on ECONNREFUSED (a refusal is deterministic on the
  network the socket is bound to; roam-time sockets are already aborted
  by reprotect)
- new dead-destination diag line (dst, fail count, cooldown) via JNI
  callback when the window escalates or hits every 100th failure
- VpnStateService: double-start guard - a second connect() for the same
  profile while a healthy attempt is in flight is ignored (four starters
  raced through here within 20 s after the 2.13.14 self-update)
- NetworkMonitor/AutoConnectManager: flap detection (>=2 kind changes in
  5 min stretches the auto-connect delay to 10 s; a Wi-Fi flap produced
  a 7-second-lived session and a wasted IKE handshake)
- hardcode diag TAGs obfuscated by R8 ("a: update: up to date" ->
  "UpdateManager: ..."), same for UpdateInstaller/VpnShortcutManager

src/frontends/android/app/build.gradle

@@ -37,8 +37,8 @@ android {
         minSdkVersion 21
         targetSdkVersion 36
 
-        versionCode 151
-        versionName "2.13.14"
+        versionCode 152
+        versionName "2.13.15"
 
         /* Base URL of the update storage, trailing slash included */
         buildConfigField "String", "UPDATE_BASE_URL", "\"${updateBaseUrl}\""