Update content url for IMA demo app

The existing one seems to be dead, and isn't https.

PiperOrigin-RevId: 222795996

demos/ima/src/main/res/values/strings.xml

@@ -17,7 +17,7 @@
 
   <string name="application_name">Exo IMA Demo</string>
 
-  <string name="content_url"><![CDATA[http://rmcdn.2mdn.net/MotifFiles/html/1248596/android_1330378998288.mp4]]></string>
+  <string name="content_url"><![CDATA[https://storage.googleapis.com/exoplayer-test-media-1/mkv/android-screens-lavf-56.36.100-aac-avc-main-1280x720.mkv]]></string>
 
   <string name="ad_tag_url"><![CDATA[https://pubads.g.doubleclick.net/gampad/ads?sz=640x480&iu=/124319096/external/single_ad_samples&ciu_szs=300x250&impl=s&gdfp_req=1&env=vp&output=vast&unviewed_position_start=1&cust_params=deployment%3Ddevsite%26sample_ct%3Dlinear&correlator=]]></string>