Make configs differing only in density siblings

Configurations differing only in density value will match the same
device configuration at runtime; therefore, they should be considered
sibling configurations in the dominator tree.

So, given configurations A and B:
1) If A has a density defined and B does not, A must never dominate B.
2) If A and B both have densities defined, neither must dominate the
   other.

Previous behavior:
<default>
  sw600dp-v13
    sw600dp-hdpi-v13
       sw800dp-hdpi-v13
    sw800dp-xxhdpi-v13

New behavior:
<default>
  sw600dp-v13
    sw600dp-hdpi-v13
      sw800dp-hdpi-v13
      sw800dp-xxhdpi-v13

Bug: 167944889
Test: aapt2_tests
Merged-In: Ie9fba4c4b74af2b3bbf8fc6432539a99f8647634
Change-Id: Ie9fba4c4b74af2b3bbf8fc6432539a99f8647634