Implement getAssets to fix crashing test

UiccCardApplicationTest crashes when run on aosp_walleye-userdebug.
Implementing getAssets with the default AssetManager (this is final and
unmockable) fixes the problem.

Bug: 29579544
Test: verify that UiccCardApplicationTest now passes
Change-Id: I6786ca44b7acef710710b539b97507204b904fe3