Guard against incorrect context use.

Instead of just having random tests fail, fail all the tests with a
slightly more useful message.

Also remove all the code around acquiring and contention, now that
we have a test rule in place for TestableContext, we can just use
that for cleanup and have a simple copy-on-write provider.

Test: runtest -x frameworks/base/tests/testables && runtest systemui
Change-Id: I907da23730a4a96cfa2bb112100a06980f01b078
Fixes: 37302051