Create a new product variable to gate blueprint files

Users can use this feature by
1. Setting PRODUCT_INCLUDE_TAGS += <val> in their product mk files
2. Set
```
blueprint_packge_includes {
  match_al: ["<val>"],
}

other_module_type {name: foo}
other_module_type {name: bar}
```

bar and foo will be included if and only if <val> is set

Test: Unit tests in blueprint
Test: TH

Change-Id: I32eed4e3b5ac47fb565c62d13d8881fa984c86f4
diff --git a/android/config.go b/android/config.go
index 4a64b5b..9d9ab30 100644
--- a/android/config.go
+++ b/android/config.go
@@ -1111,6 +1111,10 @@
 	return append([]string(nil), c.productVariables.NamespacesToExport...)
 }
 
+func (c *config) IncludeTags() []string {
+	return c.productVariables.IncludeTags
+}
+
 func (c *config) HostStaticBinaries() bool {
 	return Bool(c.productVariables.HostStaticBinaries)
 }