Camera: Remove redundant RequestThread wait during flush

If RequestThread::clear() is called right before the
mRequestSignal.waitRelative(), an extra 50ms latency is incurred.

Creating a condition for the conditional variable fixes the issue.

Test: Camera CTS
Bug: 228419719
Change-Id: Ibc8d2c9e2c98d68b73d2442118694c7a33e9d3b5