Add MediaCodec.CryptoInfo.getPattern

Allows MediaParser and MediaExtractor clients to know the
encrpytion pattern of the parsed/extracted media. Without
the getter, only MediaCodec can know the pattern, which
impossibilitates the use of app-bundled decoders.

Bug: 158743263
Test: atest CtsMediaTestCases:MediaCodecTest.testCryptoInfoPattern
Change-Id: Iaf77c8ecafad093cfa434a9ac31314895a44e78f