Export header libraries to make as static libraries

Header libraries were building an empty .a file but setting
LOCAL_MODULE_CLASS := SHARED_LIBRARIES.  Make the static and shared
checks in libraryDecorator.AndroidMk explicit.

Bug: 35228396
Test: builds, examine out/soong/Android-${TARGET_PRODUCT}.mk
Change-Id: I5653ae92bb41d440d8c3ff4b6d509be188feb961

cc/androidmk.go

@@ -81,14 +81,14 @@ func (c *Module) AndroidMk() (ret android.AndroidMkData, err error) {
 }
 
 func (library *libraryDecorator) AndroidMk(ctx AndroidMkContext, ret *android.AndroidMkData) {
-	if !library.static() {
+	if library.shared() {
 		ctx.subAndroidMk(ret, &library.stripper)
 		ctx.subAndroidMk(ret, &library.relocationPacker)
 	}
 
-	if library.static() {
+	if library.static() || library.header() {
 		ret.Class = "STATIC_LIBRARIES"
-	} else {
+	} else if library.shared() {
 		ret.Class = "SHARED_LIBRARIES"
 	}
 
@@ -118,7 +118,7 @@ func (library *libraryDecorator) AndroidMk(ctx AndroidMkContext, ret *android.An
 		return nil
 	})
 
-	if !library.static() {
+	if library.shared() {
 		ctx.subAndroidMk(ret, library.baseInstaller)
 	}
 }