Cleanup instrumentation_levels and instrumentation_stubs_installed

We had instrumentation_levels and instrumentation_stubs_installed which
were kind of similar but slightly different in what they actually
represent. Their meaning also changed with the recent changes to avoid
instrumentation stubs. They were used sometimes incorrectly in the code.
This CL:
1. Renames instrumentation_stubs_installed to run_exit_hooks
2. Renames the instrumentation level to not refer instrumentation stubs
3. Fixes a few places that should have checked for the instrumentation
   level but checked for instrumention_stubs_installed.

Bug: 206029744
Test: art/test.py
Change-Id: I20a6e9442661a6465c92321904c846d35ebb1e53
12 files changed