ResXmlEncoders: replaced findNonPositionedSubstitutions() by more flexible findSubstitutions().

Also rewritten some bits.
Decode "%d %" with formatted="false". It's not necessary for current aapt, but it's more consistent with "%d % ".
Cleaned up the code related to multiple substitutions in xml strings.
Now one non positional substitution and one positional is detected as "multiple non positional".