BinderProxyTest: fix failures

PowerManager was changed to use caching, and so this
test could not use it to reliably make binder calls.

I've refactored the test to use an API in ActivityManager
instead, which could not be cached, and am following up
separately to add presubmit coverage.

Bug: 348072409
Test: atest android.os.BinderProxyTest
Change-Id: I4a68e0e797c2489704ece144461461ed48ef3bae