Fix isCryptoSchemeSupported's return value.

If mimeType is not defined, isCryptoSchemeSupported will always
return false.

Test: Netflix on Fugu

Test:
ANDROID_BUILD_Troid-gts/tools/gts-tradefed run gts -m GtsMediaTestCases
--test com.google.android.media.gts.MediaDrmTest#testWidevineSanity

bug: 35363342
Change-Id: I579dd6d41d17c1520404a5b45baae3f730f8245a