[res] Make TargetResourcesContainer valid after an error

ApkResourcesContainer class has a variant that is either a zip
file object, or a full blown AssetManager with ApkAssets with
the same zip file inside. But if transition from one to the
other fails, it could end up in a state with neither, causing a
crash if the users try accessing it.

This change ensures that:
1. The zip file object is only moved into ApkAssets if the
   loading succeeds

2. If any part of the further initialization of ResState fails,
   we take the zip file out and put it back into the variant

+ Add unit tests for both ApkAssets and libidmap2 to ensure
  they don't crash in any of those scenarios (they used to)

+ Enable root tests in libidmap2 as they were never running

Flag: EXEMPT bugfix
Bug: 381108280
Test: atest libandroidfw_tests idmap2_tests + boot
Change-Id: I8f4803fdf03a41ba7a6892e6aed07f04b77788ce