Android Layout Background Issue

First of all, make sure that your original image looks good so you're not just getting the problem from there.
Then, in your onCreate() method, do:

code1:

getWindow().setFormat(PixelFormat.RGBA_8888);
getWindow().addFlags(WindowManager.LayoutParams.FLAG_DITHER);

And to load your image explicitly as a 32-bit image (RGBA-8888 configuration) add the following where you load your views:

code2:

BitmapFactory.Options options = new BitmapFactory.Options();
options.inPreferredConfig = Bitmap.Config.ARGB_8888;
Bitmap gradient = BitmapFactory.decodeResource(getResources(), R.drawable.gradient, options);

findViewById(R.id.main).setBackgroundDrawable(new BitmapDrawable(gradient));

Comparison between different approaches: (these are all screenshots from the resulting application)

My source images (64 colors to the left, 24 bit to the right):
image1 and image2:

1: Raw 64-color image (image1) set as background from layout XML:

2: The same image (image1), using code1:

3: The same image (image1) using both code1 and code2:

4: image2, loaded with code1 and code2 (in this case the dithering is not really important as both the source and destination use 8 bits per color):

Notice how the resulting artifacts in image 3 already exists in the original image.