Use stub-annotations to provide NonNull for hwbinder-stubs-docs

The annotations in srcs aren't doing anything because they are all
@hide.  The only thing keeping hwbinder-stubs-docs compiling is
a hack in metalava that treats anything with
hwbinder-stubs-docs-stubs.srcjar.rsp in its input files as
effectively having annotations_enabled: false.

Remove the unnecessry srcs, and add stub-annotations, which contains
a stub definition for android.annotation.NonNull, to hwbinder.stubs.
None of the rest of the annotations in srcs are used in the
generated stubs.

Test: m hwbinder.stubs
Change-Id: I7ec9afb41ea9617f25d1dffd3382fed90997b751

StubLibraries.bp

@@ -422,10 +422,6 @@ droidstubs {
     name: "hwbinder-stubs-docs",
     srcs: [
         "core/java/android/os/HidlSupport.java",
-        "core/java/android/annotation/IntDef.java",
-        "core/java/android/annotation/IntRange.java",
-        "core/java/android/annotation/NonNull.java",
-        "core/java/android/annotation/SystemApi.java",
         "core/java/android/os/HidlMemory.java",
         "core/java/android/os/HwBinder.java",
         "core/java/android/os/HwBlob.java",
@@ -451,6 +447,7 @@ droidstubs {
 java_library_static {
     name: "hwbinder.stubs",
     sdk_version: "core_current",
+    libs: ["stub-annotations"],
     srcs: [
         ":hwbinder-stubs-docs",
     ],