Keep signatures of presigned prebuilt test apps.

Skip the JNI uncompress step for presigned prebuilt test apps. They
don't need it, and they can invalidate the signature. Also, fix the
install partition of prebuilt test apps.

Fixes: 143472191
Test: app_test.go
Test: Converted CtsShimPrivUpgradePrebuilt to bp and checked the
final output apk signature.

Change-Id: Ied7d3576b5db4de40a9ca9e388336229d07212f5