Add filegroup support to notice property.

Also, replace all notice file references with the parent directory
pattern(../) with filegroups. This new version has
ALLOW_MISSING_DEPENDENCIES support, so shouldn't break branches that
don't contain the libwinpthread project.

Bug: 118899640
Test: Manual build + forrest runs on previously failed branches.
Change-Id: I13e70e8dab547f82c1c8f15eccc7ae116e480ad5
diff --git a/Android.bp b/Android.bp
index be9cf2a..9711c11 100644
--- a/Android.bp
+++ b/Android.bp
@@ -436,7 +436,7 @@
             src: "prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/x86_64-w64-mingw32/lib/libwinpthread.a",
         },
     },
-    notice: "../../prebuilts/gcc/linux-x86/host/x86_64-w64-mingw32-4.8/licenses/mingw-w64-svn-r5861/mingw-w64-libraries/winpthreads/COPYING",
+    notice: ":mingw-libwinpthread-notice",
 }
 
 toolchain_library {