fix race in ProcessState::getThreadPoolMaxTotalThreadCount

The race was introduced in https://r.android.com/3107366. Triggering the
race requires calling `getThreadPoolMaxTotalThreadCount` concurrently
with `startThreadPool`, which is a bad practice, but, we shouldn't
crash.

Bug: 355739944
Test: ran https://r.android.com/3207755 test for hours
Change-Id: Iee9a99a213474f5b1a398e703b2af585ece6828f