Move package signing after file stripping

For dex preopt and JNI library extraction, we should remove the
classes.dex and .so files before we sign the APK so that there
isn't an entry in the manifest.

Prebuilt APKs which are pre-signed will simply not have the files
removed. This may cause some system.img bloat, but signature checks make
this necessary.

Bug: 20247329
Change-Id: I4742d1aa3aa64ab5aea2264304cb8c0bea24f784