[res] Fix ResourcesManagerTest

1. Override the correct overload to mock the AssetManager
creation

2. Fix the test that checks for the cached resources object
(should be ResourcesImpl, not Resources)

Bug: 313502732
Test: atest ResourcesManagerTest
Change-Id: I0fd4fb75cbb7a01299b65600ebc295a97bfd5b15
2 files changed