Make ndk_prebuilt_*_stl's libDir relative to module

Currently, we have ~4 of these modules which are all defined in
prebuilts/ndk. However, using ctx.ModuleDir() instead has the following
advantages
- makes bp2build simpler since we do not need to relativize this path
- prevents soong modules from reaching into another directory (as the
  test setups were doing).

Test: m nothing
Change-Id: I780e2564cb37ebf4b800f0cd184789f3fc6f2fc8

cc/testing.go

@@ -558,7 +558,7 @@ var PrepareForTestWithCcBuildComponents = android.GroupFixturePreparers(
 	// This includes files that are needed by all, or at least most, instances of a cc module type.
 	android.MockFS{
 		// Needed for ndk_prebuilt_(shared|static)_stl.
-		"prebuilts/ndk/current/sources/cxx-stl/llvm-libc++/libs": nil,
+		"defaults/cc/common/current/sources/cxx-stl/llvm-libc++/libs": nil,
 	}.AddToFixture(),
 )