Only check sibling config values to dedupe

Currently more config values are being kept than necessesary. For
example, given config values for a string resource:

Config: default     Value: "keep"
Config: ldrtl       Value: "dedupe"
Config: ldrtl-night Value: "dedupe"
Config: land        Value: "keep2"

The ldrtl-night config value will fail to be removed despite being
equivalent to the ldrtl value. This is because the value for the land
configuration is not equivalent to the ldrtl-night value. Instead of
checking that every compatible config value not related by dominance
should have quivalent values, only check sibling config values within
the dominator tree.

Bug: 137230022
Test: aapt2_tests
Change-Id: I965365d1a9433ae595eab48d82837ac102148334