Camera: Set default values of HEIF gainmap fields

Some of the newly introduced HEIC composite stream fields
related to HEIF gainmap support were not given initial
default values.

Flag: com.android.internal.camera.flags.camera_heif_gainmap
Bug: 362608343
Test: Successful build
Change-Id: I60bd1a03f9e40ab75d215e68a54a2608df414186