Fix generateReleaseJavadoc "unmappable character for encoding ASCII" error.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=131714051

library/build.gradle

@@ -77,6 +77,7 @@ android.libraryVariants.all { variant ->
         options {
         options {
             links "http://docs.oracle.com/javase/7/docs/api/"
             links "http://docs.oracle.com/javase/7/docs/api/"
             linksOffline "https://developer.android.com/reference","${android.sdkDirectory}/docs/reference"
             linksOffline "https://developer.android.com/reference","${android.sdkDirectory}/docs/reference"
+            encoding = 'UTF-8'
         }
         }
         exclude '**/BuildConfig.java'
         exclude '**/BuildConfig.java'
         exclude '**/R.java'
         exclude '**/R.java'