AAPT2: Merge binary XML file attributes

AAPT2 generates extra versions of XML binaries based on the sdkVersion
that attributes were introduced, up to v21. This should only happen for
sdkVersions that are > minSdkVersion, as everything else can be included
in the main configuration (e.g. "layout" instead of "layout-v4").

This change handles the merging of XML binary sdkVersions that were created
implicitly.

Bug: 30705740
Change-Id: I854843dd6dbaca52896cea0c24c70c900cec3e8e