AAPT2: Accept density config as default

When a resource does not have a default config value defined, and the
resource only has config values defined for language translations, that
resource should be removed; however, if the resource also contains a
density specific config value, the resource should not be removed.

This is because a density specific value should have the same content as
what the default value would have, just higher or lower quality.

Bug: 76413048
Test: aapt2_tests
Change-Id: I52fe37a7aa3aefb694d3af6c8adfd6269fa26f54