apex.go: Add support for diffing content of APEX with whitelist file

So far a very basic version.

In case there is a diff, soong will emit an error message with a command
to resolve the diff. Example:

New unexpected files were added to com.android.apex.cts.shim.v3. To fix the build run following command:
system/apex/tools/update_whitelist.sh system/apex/shim/build/default_shim_whitelist.txt out/soong/.intermediates/system/apex/shim/build/com.android.apex.cts.shim.v3/android_common_com.android.apex.cts.shim.v3/com.android.apex.cts.shim.v3-content.txt

Test: m checkbuild
Bug: 139125405
Change-Id: I57e694f394c56105dc7363a684983605c7bf5e7d