Do not run zipalign when building APK (except prebuilts).

This changes the build system to no longer run zipalign except for
those prebuilts which do not need to re-signed.

Running zipalign is no longer needed because signapk takes care of
alignment. For those prebuilts which do not need to be re-signed
signapk is not invoked and thus zipalign is still needed because the
prebuilts may not have been aligned properly. This exemption will go
away once prebuilts are properly aligned.

Bug: 25794543
Change-Id: I31f10af80b9baa3c9481cb36f41e5e5046e852f9