PackageManager plumbing to support non-staged APEX installs

This is a minimal set of changes to support non-staged APEX installs in
PackageManager and friends.

In a nutshell, PackageManager simply passes the APEX around and
delegates the actual installation to ApexManager, which will do the
following:

* Check that outer .apex container is well formed.
* Check that outer .apex container is signed with the correct cert.
* Check that install is not a downgrade.
* Call into apexd to perform an actual install.
* Update internal data structures used by PackageManager's query APIs
  (getPackageInfo, getAppliationInfo, getInstalledPackages, etc.).

In addition this CL implements the following set of policies around
non-staged APEX install (some of them are likely to be relaxd in T):

* Multi apk install of both APEXes and APKs is not supported.
* Rollback enabled non-staged install of an APEX is not supported.

Since there are a bunch of host-side tests that expects
`adb install foo.apex` to perform a staged install, I've temporarily
added `--force-non-staged` option to provide a way to trigger a
non-staged APEX update via adb. Once we've migrated all the test cases
to explicitly mention `--staged` flow, we can remove this flag and make
APEX adb install follow the same logic as APK one (if a developer wants
to perform a staged install, they need to explicitly add --staged flag).

Test: adb install --force-non-staged shim.apex
Test: atest CtsStagedInstallHostTestCases
Test: atest CtsRollbackManagerHostTestCases
Bug: 187864524
Bug: 188714038
Change-Id: Ia1fb1d07dee465bae572321a510ae78b56c35614
Merged-In: Ia1fb1d07dee465bae572321a510ae78b56c35614