Fix flakiness in TrustedPresenationCallback tests

On certain devices, these tests can fail since they are expecting all
pixels in the SC to be visible. Some configurations show content above
the SC content that can't be prevented. Instead, drop the threshold
since it doesn't need to be 100% visible to test
TrustedPresentationCallback.

Test: TrustedPresentationCallbackTest
Bug: 279957280
Change-Id: I4709115278135bde463eb4ed2de250f5041ea3e3