codec2 plugin: misc fixes

- add null check for processing queried parameters
- use comp->intf() instead of creating separate intf
- createBlockPool() now correctly registers filter components

Bug: 184312883
Test: adb shell setprop debug.codec2.force-sample-plugin true
Test: atest CtsMediaTestCases:DecoderTest
Change-Id: I77f0ae6fe3f42c6fe5d71ff8d7ad99cf279959d7