Create a new file inside res/drawable folder and save it as triangle.xml

<?xml version="1.0" encoding="utf-8"?>

<layer-list xmlns:android="http://schemas.android.com/apk/res/android" >
    <item> 
      <rotate android:fromDegrees="45"
            android:toDegrees="45" 
            android:pivotX="-40%" 
            android:pivotY="87%" > 
           <shape android:shape="rectangle" >
                 <stroke android:color="#000000"
                      android:width="10dp"/> 
                 <solid android:color="#000000" />
          </shape> 
      </rotate>
   </item>
 </layer-list>