Add comprehensive icon asset and size verification through BiometricPrompt transitions

Refactors PromptViewModelTest to use Kosmos for mocks, adds rearfps test
case to PromptViewModelTest, and adds comprehensive icon asset and size
verification through all BiometricPrompt transitions:
1) idle -> authenticating
2) authenticating -> error -> clear error
3) authenticating -> authenticated
4) authenticating -> pending confirmation
5) authenticated but pending confirmation -> explicitly authenticated

and across all modalities and configurations:
1) rear FPS
2) SFPS -> a) rear display mode b) folded configuration c) unfolded configuration
3) UDFPS
4) face auth
5) co-ex implicit auth
6) co-ex explicit auth

This will replace BiometricPromptIconOnlyScreenshotTest
with unit tests, because we have no way to reliably screenshot lotties
given the current screenshot infrastructure.

Flag: TEST_ONLY
Test: atest PromptViewModelTest
Fixes: 335757373
Fixes: 348889898
Change-Id: I22002da4dcb71f96f582afe617c56e8fa6a42673