Use ArchType in dexpreopt config

Make ArchType implement the encoding.TextMarshaller and
encoding.TextUnmarshaller interfaces so that it can be used
as a value in the dexpreopt config structs that are passed
through JSON files.

Test: m checkbuild
Change-Id: Ie4c12443e7ee5fe43f42d5403bcde12d62f617e2

dexpreopt/dexpreopt_test.go

@@ -76,7 +76,7 @@ var testModuleConfig = ModuleConfig{
 	OptionalUsesLibraries: nil,
 	UsesLibraries:         nil,
 	LibraryPaths:          nil,
-	Archs:                 []string{"arm"},
+	Archs:                 []android.ArchType{android.Arm},
 	DexPreoptImages:       []string{"system/framework/arm/boot.art"},
 	PreoptExtractedApk:    false,
 	NoCreateAppImage:      false,