Camera3: Be busy as soon as work arrives.

RequestThread's internal busy flag (mPaused) was not being immediately
set when new work was submitted to it. This allowed for a race
condition where a capture() followed by an immediate
waitUntilDrained() would immediately return from the wait.

Set the mPaused flag to false immediately in capture() and
setStreamingRequest() to avoid this, instead of waiting until the end
of the next RequestThread iteration.

Bug: 10531739
Change-Id: I54a79fe5361d527ec717f41ad805e9b319a48cd8