[SB] Update StatusBarSignalPolicyTest with more Kosmos usage.

- Use `useUconfinedTestDispatcher()` so we don't need `runCurrent`
  everywhere
- Use more values from Kosmos
- Set `underTest` as a Kosmos fixture
- Use `Kosmos.runTest`

Also updates to org.mockito.kotlin for `mock`.

Bug: 291321279
Flag: TEST_ONLY
Test: atest StatusBarSignalPolicyTest
Change-Id: Icf537d0d3eed3e4ac87f5a8423823b40a9b84cd9