Android: use gradle repo mavenCentral() instead of jcenter()

jcenter() is deprecated and will be shutdown by next year [1], thus
the replacement mavenCentral() could be used instead.

[1] https://blog.gradle.org/jcenter-shutdown

Pick-to: 6.2
Change-Id: Ic9d1c15d657f23712ee4c866d5c1a45706353429
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/android/templates/build.gradle

@@ -1,7 +1,7 @@
 buildscript {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 
     dependencies {
@@ -11,7 +11,7 @@ buildscript {
 
 repositories {
     google()
-    jcenter()
+    mavenCentral()
 }
 
 apply plugin: 'com.android.application'