AAPT2: Remove signatures from multi-APK artifacts.

Remove signer files (.SF and public key) as well as the original
manifest file from any output artifacts. Since the artifacts are not
signed, we want to remove the original signature and manifest. The APK
signer tool will add these back later.

Updated the context wrapper to enable verbose mode to be cleared. This
lets us have more specific log messages when splitting an APK as at this
point the artifacts and filters are more interesting than the files removed.

Test: Manually split an APK and verified the filles were removed while
      leaving the other META-INF entries.
Test: Unit tests

Change-Id: Ia59993b5570f802130c6e3ba42398a00821061ea