AAPT2: Refactor flags into commands

Refactors the flag based command invocation into classes that make using
subcommands easier.

Test: manual tests of printing
Change-Id: Ic8df6af0be30db552e32150afebecbfeec7e1075
diff --git a/tools/aapt2/Android.bp b/tools/aapt2/Android.bp
index 7a33de0..2ecf25b 100644
--- a/tools/aapt2/Android.bp
+++ b/tools/aapt2/Android.bp
@@ -15,6 +15,7 @@
 //
 
 toolSources = [
+    "cmd/Command.cpp",
     "cmd/Compile.cpp",
     "cmd/Convert.cpp",
     "cmd/Diff.cpp",
@@ -124,7 +125,6 @@
         "ConfigDescription.cpp",
         "Debug.cpp",
         "DominatorTree.cpp",
-        "Flags.cpp",
         "java/AnnotationProcessor.cpp",
         "java/ClassDefinition.cpp",
         "java/JavaClassGenerator.cpp",