Fix thread priority after boosting.

The previous CL wrongly restores all priority to nice 0.

This CL changes two things:

1. It remembers and restores the previous priority.
2. If the thread has priority of < -2. Do not set it to -2.

BUG: 29930920
Change-Id: I927e060d789c9bbf92f6a166f44317d9db0f5205