AAPT2: Fix issue with enums and integer attributes

When an attribute had the format "enum|integer", and a max or min
allowed value set, any value set for this attribute would have its
enum symbol's value checked against the valid integer range.

This would lead to the following:

android:numColumns="autofit"

being interpreted as an integer value of -1, which violated the minimum
expected value for numColumns, which was 0.

Bug: 62358540
Test: make aapt2_tests
Change-Id: I3150410448a533d3595a08ac6b2966264db874d8