Enables child APKs, fixes a package parsing bug, and adds tests.

1. Enables child APKs by default. (This is primarily so the tests work, but I'm
open to other options.)

2. Fixes bug where child packages would steal their parent's components of the
same type of the parent.

(e.g. if the child has a service, it would steal all of the parent's services,
but only reproduced for more complicated manifests)

This changes the way component parse args are cached and reused. Previously
they were cached in a field and reset each time a new package was scanned.  But
with child APKs, a single manifest can have multiple packages.  Now, the
lifetime of the cache is limited to a single <application> element, and rather
than being a member field, it's passed explicitly as a parameter so its
lifetime can be explicitly controlled without the need to remember to null it.

3. Adds tests for the bug PackageParser

These tests just test the parsing component without doing full installation.
And once b/62385737 (failing tests) are fixed, we can add new
PackageManagerTests.

Change-Id: Iccd148b571af86daab48d7a9e28ab6d636b1ad00
Fixes: 62032885
Test: Tested a large app with a child package and the previous issue did not reproduce after this change.