Update Gradle version from 4.1 to 4.4

In Gradle 4.4, it is a bug to resolve a configuration before the lint task is
created ([see [] Therefore, to upgrade
gradle version, we need to change the "generateJavadoc" task to remove using
files() call during initialization phase, but change move this to doFirst()
instead.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=190634090

RELEASENOTES.md

@@ -2,6 +2,8 @@
 
 ### dev-v2 (not yet released) ###
 
+* Gradle: Upgrade Gradle version from 4.1 to 4.4 so it can work with Android
+  Studio 3.1 ([#3708](https://github.com/google/ExoPlayer/issues/3708)).
 * Optimize seeking in FMP4 by enabling seeking to the nearest sync sample within
   a fragment. This benefits standalone FMP4 playbacks, DASH and SmoothStreaming.
 * Optimize seeking in FMP4.

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-all.zip

javadoc_library.gradle

@@ -20,8 +20,6 @@ android.libraryVariants.all { variant ->
         description = "Generates Javadoc for the ${javadocTitle}."
         title = "ExoPlayer ${javadocTitle}"
         source = variant.javaCompile.source
-        classpath = files(variant.javaCompile.classpath.files,
-	                  project.android.getBootClasspath())
         options {
             links "http://docs.oracle.com/javase/7/docs/api/"
             linksOffline "https://developer.android.com/reference",
@@ -30,6 +28,12 @@ android.libraryVariants.all { variant ->
         }
         exclude "**/BuildConfig.java"
         exclude "**/R.java"
+        doFirst {
+            classpath =
+                files(
+                    variant.javaCompile.classpath.files,
+                    project.android.getBootClasspath())
+        }
         doLast {
             copy {
                 from "src/main/javadoc"