Camera: fix use after free in sensor timestamp

The metadata object might be overriden later and has it memory
re-allocated; hence snaping the sensor timestamp value before
we call into any method that might change the metadata.

Test: build
Bug: 150944913
Merged-In: I0f944fc9133d3ab279859f20236d956d7ca338f8
Change-Id: I0f944fc9133d3ab279859f20236d956d7ca338f8
(cherry picked from commit 60afc2fd)