Protect against NPE in Icon#invalidateResourcesCacheIfNeeded

It seems that in some edge case scenarios on Google Watch Faces, the
resources objects can be null.

Fixes: 438621327
Test: atest FrameworkCoreTests:IconTest
Flag: com.android.graphics.flags.use_resources_from_context_to_create_drawable_icons
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:cb178b3c328c74ec1e511fe6544576b2f02b6259
Merged-In: I50e5985da52b8edcd694010304c8d469ba35b625
Change-Id: I50e5985da52b8edcd694010304c8d469ba35b625