Don't compile all splits when installing one.

This patch consists of two broad changes :
- don't "force" dex2oat when installing a new app. this should never
  be necessary because we will always compare checksums.

- when staging a new install, we "inherit" (hard link) all compiled
  oat files from the previous install. this will ensure that we compile
  only those files that have changed, and not all of them

bug: 20889739
Change-Id: I3e14335f3bcfe76d1d24d233f53a728a6d90e8a1