camera: Drop pending preview for enableZsl shots

If the device support ZSL, drop pending preview buffers to reduce
the chance the rendering preview frames newer than the still frame
during takePicture().

Test: CTS
Bug: 67497723
Change-Id: I5f253a402a6302d31777ad4ca2878ef0d0d1ae44
(cherry picked from commit c75eb9b126b2c6b7fc8f47d8ae4e4ac15f6a176e)