Enhance documentation in mainline stubs

Metalava has a feature to "enhance documentation" which is mostly based
on auto-generating documentation text based on source-retention
annotations. This is turned on automatically for doc stubs, but here
we are enabling it for module's jar stubs as well.

The point of this is to be able to replace the module source code with
module jar stub source code when generating doc stubs -- something
previously not possible because most source-retention annotations are
not present in the jar stubs.

In order for the enhancement to work properly, metalava needs to be
passed the annotation source code rather than the annotations library.
This is because of various javadoc tags controlling what documentation
is generated, e.g. @paramDoc, @returnDoc etc.

Bug: 186197911
Test: manually inspect module stub source code is enhanced
Change-Id: I3fda4cd64e49c1da33fe614c30aafd9fa0dba3b9

sdk/ModuleDefaults.bp

@@ -30,7 +30,8 @@ mainline_stubs_args =
     "--hide SdkConstant " +
     "--hide Todo " +
     "--hide Typo " +
-    "--hide UnavailableSymbol "
+    "--hide UnavailableSymbol " +
+    "--enhance-documentation "
 
 // TODO: modularize this so not every module has the same list
 framework_packages_to_document = [
@@ -76,8 +77,14 @@ mainline_service_stubs_args =
 java_defaults {
     name: "framework-module-common-defaults",
 
-    // Additional annotations used for compiling both the implementation and the
-    // stubs libraries.
+    // Use the source of annotations that affect metalava doc generation, since
+    // the relevant generation instructions are themselves in javadoc, which is
+    // not present in class files.
+    api_srcs: [":framework-metalava-annotations"],
+
+    // Make the source retention annotations available on the classpath when compiling
+    // the implementation library. (This should be in impl_only_libs but some modules
+    // use these defaults for java_library, sigh.)
     libs: ["framework-annotations-lib"],
 
     // Framework modules are not generally shared libraries, i.e. they are not