CodecCapabilities: Fix validFeatures().

This test api returns an array with the same size as getValidFeatures()
but having null objects.
It is hide and not called by any tests before.
The issues is detected by the newly added NativeAMediaCodecInfoTest.

Bug: 363320865
Test: NativeAMediaCoecInfoTest
Change-Id: Ib1a4ceb77e04e607af59bab244cc36b9fb16bbad