Add ability to build unbundled builds.

This adds ability to build using jcenter instead of local prebuilts
directory. This is needed for special unbundled checkouts of support
library that use remote maven to get all of its dependencies.

Bug: 72221688
Test: local build of support lib
Change-Id: Icec568e27caa745bbf20f82c179509de1d4c2485
diff --git a/emoji-compat/build.gradle b/emoji-compat/build.gradle
index 0da67c7..f21cd6f 100644
--- a/emoji-compat/build.gradle
+++ b/emoji-compat/build.gradle
@@ -24,8 +24,12 @@
 }
 
 buildscript {
+    boolean unbundleBuild = (new File("unbundled-build")).exists()
     repositories {
         maven { url '../../../prebuilts/tools/common/m2/repository' }
+        if (unbundleBuild) {
+            jcenter()
+        }
     }
 
     dependencies {