C2SoftVpxEnc: Correct minimum level setting code

When the encoder is given a level lower than required for the
current configuration, plugin wasn't setting it correctly.

Bug: 151430764
Test: Uncomment the parameter for VP9 in the CTS test
 atest android.mediav2.cts.EncoderProfileLevelTest

Change-Id: I123c8d00b6b09858b732d1f741cf92e241067a79