ProCamera: Fix waitForFrameBuffer not handling multiple outstanding frames

If the CpuConsumer triggered multiple onFrameAvailable callbacks in between
a single waitForFrameBuffer call, the old code would only handle 1 callback.

This meant on two subsequent waitForFrameBuffer calls the second would always
timeout when two buffers were already available to be unlocked.

Bug: 8238112
Change-Id: Ibefca35005ac5c408e5ada97ec4a4344a9e3e497