Fix incorrect proto apk loading

If a proto apk is missing resources.pb, AAPT2 would fail to detrmine the
apk format because of an incorrect check. This fixes that check and
removes the checking fuction from the LoadedApk header.

Bug: 117820549
Test: removed resources.pb from proto apks and confirmed that loading in
apks still works

Change-Id: If3628a821e7b59c7dfcbefb502b6080be083cec1