Camera: Fix CPU hogging when flush() is called after device goes idle

If flush() is called after the request thread goes idle, we still need
to reset the mRequestClearing flag. Otherwise, the requestThread will
go onto a busy loop.

Test: Camera CTS, Observe CPU trace when exiting camera
Bug: 232711266
Change-Id: I25c191e7df2a788ad34ab137b39da8d5031beb0a