Add back jcenter() to repositories

The buildscript entry is required by
com.android.tools.build:gradle:4.0.1.

For other projects, it appears that org.checkerframework is
in jcenter.

Without this change building with gradlew is broken, at least.

#minor-release

PiperOrigin-RevId: 366222799

build.gradle

@@ -14,6 +14,7 @@
 buildscript {
     repositories {
         google()
+        jcenter()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:4.0.1'
@@ -23,6 +24,7 @@ buildscript {
 allprojects {
     repositories {
         google()
+        jcenter()
     }
     if (it.hasProperty('externalBuildDir')) {
         if (!new File(externalBuildDir).isAbsolute()) {