Write files in a temporary directory

Avoid writing test files in the build output directory which fails when
run with Bazel. This happens because Bazel's sandboxing environment
ensures that the test's working directory is unwritable.

See https://docs.bazel.build/versions/main/sandboxing.html for more
information.

Bug: 209687942
Test: atest --bazel-mode aapt2_tests
Test: atest aapt2_tests

Change-Id: I0fce701bbac255c3e308ad5c85aa27230c3b620b