AAPT2: Compile --zip flag

Added a --zip flag similar to --dir that allows resources to be passed
into "aapt2 compile" using a zip file.

Also refactored Compile.cpp to be easier to mock and test in the future.

Bug: 74574557
Test: aapt2_tests
Change-Id: Idb90cb97e23a219525bdead38220cbf7bc6f3cab