Run platform_library-docs always with -source 1.8

Test: m -j USE_R8=true EXPERIMENTAL_USE_OPENJDK9=true
platform_library-docs

Bug: b/110824610
Change-Id: Ifa69eddb128a31cb4c371745b5c68bef116e7afa
diff --git a/frameworks/PlatformLibrary/Android.bp b/frameworks/PlatformLibrary/Android.bp
index 875b598..e78470f 100644
--- a/frameworks/PlatformLibrary/Android.bp
+++ b/frameworks/PlatformLibrary/Android.bp
@@ -17,4 +17,5 @@
     srcs: [
         "java/**/*.java",
     ],
+    java_version: "1.8",
 }