Makes rememberViewModel's coroutine context configurable

There are two ways to control it:
- WithConfiguredRememberViewModels wraps a composable hierarchy and
  makes every rememberViewModel within it use the provided
CoroutineContext
- rememberViewModel takes a coroutineContext parameter. If this is
  passed in, in addition to the one from
WithConfiguredRememberViewModels, the passed-in one wins

Bug: 436547539
Test: unit tests added, also tested manually with the followup CL,
making sure that activated view-models are reporting background thread
names instead of "main" when they get activated
Flag: EXEMPT no production changes in this CL

Change-Id: I1edd2dd261bf1dd3713711f1be242716b6687efe