Add "nodpi" density, and expose a bunch of density-related APIs.

Also update the DpiTest app to use nodpi images, and try to have a mode
where it turns off compatibility though it's not quite working.
diff --git a/tools/aapt/AaptAssets.cpp b/tools/aapt/AaptAssets.cpp
index 67af116..14cad2f 100644
--- a/tools/aapt/AaptAssets.cpp
+++ b/tools/aapt/AaptAssets.cpp
@@ -654,9 +654,15 @@
                                     ResTable_config* out)
 {
     if (strcmp(name, kWildcardName) == 0) {
-        if (out) out->density = 0;
+        if (out) out->density = ResTable_config::DENSITY_DEFAULT;
         return true;
     }
+    
+    if (strcmp(name, "nodpi") == 0) {
+        if (out) out->density = ResTable_config::DENSITY_NONE;
+        return true;
+    }
+    
     char* c = (char*)name;
     while (*c >= '0' && *c <= '9') {
         c++;