[Signin][Android] Use support library fragments in SigninActivity

This is a follow-up to https://crrev.com/c/980335.
This CL migrates SigninActivity and SigninFragmentBase to support
library fragments (android.support.v4.app.Fragment instead of
android.app.Fragment).

Bug: 825789, 814728
Change-Id: I76d8a5eb329e9b418d1ae1c75a8d272d2b7c6009
Reviewed-on: https://chromium-review.googlesource.com/982054Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546121}

chrome/android/java/src/org/chromium/chrome/browser/signin/SigninActivity.java

@@ -4,11 +4,11 @@
 
 package org.chromium.chrome.browser.signin;
 
-import android.app.Fragment;
-import android.app.FragmentManager;
 import android.content.Context;
 import android.content.Intent;
 import android.os.Bundle;
+import android.support.v4.app.Fragment;
+import android.support.v4.app.FragmentManager;
 import android.support.v7.app.AppCompatActivity;
 
 import org.chromium.base.Log;
@@ -51,7 +51,7 @@ public class SigninActivity extends AppCompatActivity {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.signin_activity);
 
-        FragmentManager fragmentManager = getFragmentManager();
+        FragmentManager fragmentManager = getSupportFragmentManager();
         Fragment fragment = fragmentManager.findFragmentById(R.id.fragment_container);
         if (fragment == null) {
             fragment = new SigninFragment();

chrome/android/java/src/org/chromium/chrome/browser/signin/SigninFragmentBase.java

@@ -5,11 +5,11 @@
 package org.chromium.chrome.browser.signin;
 
 import android.annotation.SuppressLint;
-import android.app.Fragment;
 import android.graphics.drawable.Drawable;
 import android.os.Bundle;
 import android.support.annotation.Nullable;
 import android.support.annotation.StringRes;
+import android.support.v4.app.Fragment;
 import android.support.v7.content.res.AppCompatResources;
 import android.text.method.LinkMovementMethod;
 import android.view.LayoutInflater;