Uncompress native libs only if they are compressed.

This changes the build system to uncompress native libraries in
preinstalled APKs only if the libraries are actually compressed.
Previously, any preinstalled APK containing native libraries was
modified by having all native libraries removed from the APK and then
re-added to the APK in uncompressed form.

Bug: 27887819
Change-Id: I3db172133e141bb6282bf3ed7070b068a826724e