Avoid android private API: variant.variantData

[zpencer]

This returns an internal class and we should avoid this call.

It makes sense just to query for variant.sourceSets
or variant.productFlavors rather than building up the source sets
ourselves.

It actually looks like the list that we create ourselves is missing a source
set during testing:
Example 1

variant.sourceSets
0 = "main"
1 = "freeapp"
2 = "x86"
3 = "x86Freeapp"
4 = "debug"
5 = "x86FreeappDebug"

sourceSetNames
 0 = "main"
 1 = "main"
 2 = "x86FreeappDebug"
 3 = "debug"
 4 = "x86"
 5 = "freeapp"

Example 2

variant.sourceSets
 0 = "main"
 1 = "retailapp"
 2 = "x86"
 3 = "x86Retailapp"
 4 = "release"
 5 = "x86RetailappRelease"

sourceSetNames
 0 = "main"
 1 = "main"
 2 = "x86RetailappRelease"
 3 = "release"
 4 = "x86"
 5 = "retailapp"