SoftXaac: Handle error cases when decoder fails to initialize

When decoder fails to initialize, it doesn't consume any bytes in
subsequent process call. Avoid this by returning an error when decoder
is not initialized before first process call.

Bug: 112194305
Test: cts-tradefed run commandAndExit cts-dev -m CtsSecurityTestCases\
 -t android.security.cts.StagefrightTest#testStagefright_c_2016_2428

Change-Id: I38ef56e8ff544567c3219c03dc6133c1ef98f3b3