Add test apex for sdkextensions

Because the tests build for 64-bit but run on 32-bit, this
requires adding a variant of derive_sdk that uses prefer32.

This apex has an SDK level of 45, which is verified in the
test. Also trigger the e2e test on changes to the apex.

Bug: 137191822
Bug: 143937446
Test: atest sdkextensions_e2e_test
Test: m
Change-Id: I88a75b4f438004ed9425e6629666cbbae1ab0882