Add RuleBuilder.Installs().String()

Add a RuleBuilderInstalls type for a slice of RuleBuilderInstalls,
and give it a String() method that returns the list of installs
in the format that is convenient for passing to Make.

Test: rule_builder_test.go
Change-Id: I2e9cd9abf4dfb0ad312d0a6662f1567baf9cd222
diff --git a/dexpreopt/dexpreopt_test.go b/dexpreopt/dexpreopt_test.go
index ecaf876..40c694f 100644
--- a/dexpreopt/dexpreopt_test.go
+++ b/dexpreopt/dexpreopt_test.go
@@ -100,7 +100,7 @@
 		t.Error(err)
 	}
 
-	wantInstalls := []android.RuleBuilderInstall{
+	wantInstalls := android.RuleBuilderInstalls{
 		{"out/test/oat/arm/package.odex", "/system/app/test/oat/arm/test.odex"},
 		{"out/test/oat/arm/package.vdex", "/system/app/test/oat/arm/test.vdex"},
 	}
@@ -141,7 +141,7 @@
 		t.Error(err)
 	}
 
-	wantInstalls := []android.RuleBuilderInstall{
+	wantInstalls := android.RuleBuilderInstalls{
 		{"out/test/oat/arm/package.odex", "/system_other/app/test/oat/arm/test.odex"},
 		{"out/test/oat/arm/package.vdex", "/system_other/app/test/oat/arm/test.vdex"},
 	}
@@ -164,7 +164,7 @@
 		t.Error(err)
 	}
 
-	wantInstalls := []android.RuleBuilderInstall{
+	wantInstalls := android.RuleBuilderInstalls{
 		{"out/test/profile.prof", "/system/app/test/test.apk.prof"},
 		{"out/test/oat/arm/package.art", "/system/app/test/oat/arm/test.art"},
 		{"out/test/oat/arm/package.odex", "/system/app/test/oat/arm/test.odex"},