Add ANDROID_JPEG_MAX_SIZE to VirtualCamera characteristics

Add it also for custom set CameraCharacteristics since
ANDROID_JPEG_MAX_SIZE doesn't have an external mapped Key (not even
hidden), but it's required for building a proper Camera3 device.

Also, improved a bit the validation and logic in setting custom metadata
in MetadataUtil, null is a valid value that doesn't need to be
validated though, causing misleading error logs.

Bug: 438779235
Bug: 371167033
Test: atest VirtualCameraCameraXTest VirtualCameraTest
CtsVirtualDevicesCameraTestCases virtual_camera_tests
Flag: android.companion.virtualdevice.flags.virtual_camera_metadata

Change-Id: If04611b7b5de514d0c09c0e51d6bbed9b45f0f07