libbinder: thread count startThreadPool spawn

When startThreadPool is called, it spawns a thread which is
not counted as part of the lazy kernel-started threads.

This was discovered in fuzzers, and the test is updated.

Bug: 286237215
Test: binderLibTest
Change-Id: Ib0fa4484576f9d296b8f57f32ae536b17e5c6497