Allow the default mode for video peek to be undefined

Forcing the default mode for video peek to be defined to on has broken
use cases for Broadcom devices.

See go/video-peek-legacy-mode for more details.

Bug: 220266078
Test: atest android.media.decoder.cts.DecoderTest
Change-Id: I0a7190c3af0b8ab5ae2f78d8f4df5864efe3b267