Asset discovery refactor

* Move asset discovery logic from JsonRpcService to new
  AssetDiscoveryManager class

* Add DiscoverAssetsOnAllOnRefresh and
  DiscoverAssetsOnAllSupportedChains APIs to
  AssetDiscoveryManager, and expose
  DiscoverAssetsOnAllOnRefresh to frontend via mojo
  API on BraveWalletService

* Add AccountsAdded event to KeyringService emitted
  when a new account is added via any method (hardware
  import, private key import, etc) that is triggers the
  AssetDiscoveryManager

* Add kBraveWalletAssetsLastDiscoveredAt and
  kBraveWalletNextAssetDiscoveryFromBlocks prefs to
  manage asset discovery state including rate limiting
  and last successful asset search per coin type /
  chain ID

android/java/org/chromium/chrome/browser/app/domain/DappsModel.java

@@ -315,6 +315,9 @@ public class DappsModel implements KeyringServiceObserver {
     @Override
     public void accountsChanged() {}
 
+    @Override
+    public void accountsAdded(int coin, String[] addresses) {}
+
     @Override
     public void autoLockMinutesChanged() {}
 

android/java/org/chromium/chrome/browser/app/domain/KeyringModel.java

@@ -329,6 +329,9 @@ public class KeyringModel implements KeyringServiceObserver {
         update();
     }
 
+    @Override
+    public void accountsAdded(int coin, String[] addresses) {}
+
     @Override
     public void autoLockMinutesChanged() {}
 

android/java/org/chromium/chrome/browser/crypto_wallet/observers/KeyringServiceObserverImpl.java

@@ -17,6 +17,7 @@ public class KeyringServiceObserverImpl implements KeyringServiceObserver {
         default void keyringReset() {}
         default void unlocked() {}
         default void accountsChanged() {}
+        default void accountsAdded(int coin, String[] addresses) {}
         default void autoLockMinutesChanged() {}
         default void selectedAccountChanged(int coin) {}
     }
@@ -76,6 +77,13 @@ public class KeyringServiceObserverImpl implements KeyringServiceObserver {
         mDelegate.accountsChanged();
     }
 
+    @Override
+    public void accountsAdded(int coin, String[] addresses) {
+        if (mDelegate == null) return;
+
+        mDelegate.accountsAdded(coin, addresses);
+    }
+
     @Override
     public void autoLockMinutesChanged() {
         if (mDelegate == null) return;