Prefer AppCompat classes

AppCompat is the preferred way to go for any app targetting
a wider range of SDKs.

Replace all activities and fragments with their AppCompat
variants and fixup method calls to use support variants.

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/java/com/wireguard/android/fragment/BaseFragment.java

@@ -1,7 +1,7 @@
 package com.wireguard.android.fragment;
 
-import android.app.Fragment;
 import android.content.Context;
+import android.support.v4.app.Fragment;
 
 import com.wireguard.android.activity.BaseActivity;
 import com.wireguard.android.activity.BaseActivity.OnSelectedTunnelChangedListener;