Fix potential deadlock in stopPreview/stopRecord.

Some camera HALs spin up a preview thread and need to wait for
the thread to exit. This can create a potential deadlock. In
stopPreview, we take the main lock. If a preview callback occurs
while the lock is held, the preview thread will block. If the
camera HAL is waiting for the preview thread to exit, this will
cause a deadlock.

This patch breaks out the preview buffer heap into a separate
mutex. This mutex is never held when the main lock is held, thus
preventing the deadlock from occuring.