The test is flaky because the decoding process in the renderer
depends on some timing from MediaCodec beyond our control and
the new keyframe added in the test is sometimes 'dropped' when
it arrives too late.
We can fix this by controlling the test progress a bit more
tightly: first rendering with the same current time until the
key frame is processed and then start increasing the time
until we've reached the end.
#cherrypick
PiperOrigin-RevId: 646064352
ada4dc982f