Guard virtual camera metadata builder with lock

The mCustomMetadata is now guarded for a better concurrency access
in the MetadataUtil builder.

Bug: 371167033
Test: atest CtsVirtualDevicesCameraTestCases
Test: atest virtual_camera_tests
Flag: android.companion.virtualdevice.flags.virtual_camera_metadata
Change-Id: Ia9963178c744b6391b40200ecb87173f203fac0c