Fix prebuilt header files to produce proper AndroidMk entries.

Necessary for modules that depend on headers through
LOCAL_HEADER_LIBRARIES in legacy Android.mk files.

Test: m nothing
Test: m nothing SOONG_CONFIG_art_module_source_build=false
  on internal master
Test: m droid checkbuild SOONG_CONFIG_art_module_source_build=false
Test: m SOONG_CONFIG_art_module_source_build=false \
        out/soong/.intermediates/prebuilts/module_sdk/art/current/sdk/prebuilt_jni_headers/linux_glibc_x86_64/prebuilt_jni_headers.a
      ar t out/soong/.intermediates/prebuilts/module_sdk/art/current/sdk/prebuilt_jni_headers/linux_glibc_x86_64/prebuilt_jni_headers.a \
        && echo ok
Bug: 202192894
Change-Id: Ied267053d1fc3563870a3faaf41d79983a22815a
diff --git a/cc/library_headers_test.go b/cc/library_headers_test.go
index 79d2291..3e448ba 100644
--- a/cc/library_headers_test.go
+++ b/cc/library_headers_test.go
@@ -43,6 +43,17 @@
 			// test if header search paths are correctly added
 			cc := ctx.ModuleForTests("lib", "android_arm64_armv8-a_static").Rule("cc")
 			android.AssertStringDoesContain(t, "cFlags for lib module", cc.Args["cFlags"], " -Imy_include ")
+
+			// Test that there's a valid AndroidMk entry.
+			headers := ctx.ModuleForTests("headers", "android_arm64_armv8-a").Module()
+			e := android.AndroidMkEntriesForTest(t, ctx, headers)[0]
+
+			// This duplicates the tests done in AndroidMkEntries.write. It would be
+			// better to test its output, but there are no test functions that capture that.
+			android.AssertBoolEquals(t, "AndroidMkEntries.Disabled", false, e.Disabled)
+			android.AssertBoolEquals(t, "AndroidMkEntries.OutputFile.Valid()", true, e.OutputFile.Valid())
+
+			android.AssertStringListContains(t, "LOCAL_EXPORT_CFLAGS for headers module", e.EntryMap["LOCAL_EXPORT_CFLAGS"], "-Imy_include")
 		})
 	}
 }