AAPT2: Implement attribute compat versioning

This change defines some hardcoded rules to degrade
attributes in newer SDKs to specific older attributes.

An attribute with a degrade rule will generate a new XML for the API
in which the attribute resulting from the degradation was introduced.

Since API 22 (Lollipop MR1), attributes are correctly ignored and do
not need to be versioned. In XML files defined for APIs 22+, the
original and degraded attributes coexist in the same XML file.

One such example is paddingHorizontal, introduced in API 26.
paddingHorizontal degrades to paddingLeft and paddingRight, which
were both introduced in API 1.

Bug: 35763493
Test: make aapt2_tests
Change-Id: I4aa8755a9ee2c0cc5afdc55c3d30093fd3a47f3d