Support aidl cpp generation

Ideally we'd calculate the headers that are written here too, but I'll
add that in a later change that actually enforces the generated header
list.

Test: mmma -j system/tools/aidl
Change-Id: Ifd2e8e8ff444b0f67270fb5156e7bf7bceddb6be

cc/builder.go

@@ -194,6 +194,7 @@ type builderFlags struct {
 	protoFlags  string
 	tidyFlags   string
 	yasmFlags   string
+	aidlFlags   string
 	toolchain   config.Toolchain
 	clang       bool
 	tidy        bool