Update pixel format metric.

Add initialization for mPixelFormat.

This update comes from: https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/av/+/24458406/comment/a395d2e6_b63acf08/

This fix has already been included in the qpr change. This change is to
sync main with qpr branch.

Bug: 270518245
Test: manual
Change-Id: I15115363e69d30378fb6723ba1207fea7a87660c