Storage: remove reflection to get raw input stream

[ajaaym]

remove reflection to get raw input stream, work towards #3929

[codecov]

Codecov Report

Merging #5515 into master will decrease coverage by 0.31%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #5515      +/-   ##
============================================
- Coverage     50.85%   50.54%   -0.32%     
+ Complexity    24192    22790    -1402     
============================================
  Files          2270     2270              
  Lines        230125   225272    -4853     
  Branches      25010    24896     -114     
============================================
- Hits         117041   113858    -3183     
+ Misses       104446   103670     -776     
+ Partials       8638     7744     -894

Impacted Files	Coverage Δ	Complexity Δ
...om/google/cloud/storage/spi/v1/HttpStorageRpc.java	1.84% <0%> (+0.02%)	1 <0> (ø)	⬇️
...m/google/cloud/vision/v1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...e/cloud/vision/v1p4beta1/ImageAnnotatorClient.java	58.69% <0%> (-3.31%)	15% <0%> (-4%)
...e/cloud/vision/v1p3beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
...e/cloud/vision/v1p2beta1/ImageAnnotatorClient.java	47.05% <0%> (-2.95%)	9% <0%> (-2%)
.../java/com/google/cloud/speech/v1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...om/google/cloud/speech/v1p1beta1/SpeechClient.java	48.57% <0%> (-2.78%)	10% <0%> (-2%)
...d/webrisk/v1beta1/WebRiskServiceV1Beta1Client.java	63.41% <0%> (-2.5%)	12% <0%> (-3%)
...ogle/cloud/texttospeech/v1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
...cloud/texttospeech/v1beta1/TextToSpeechClient.java	55.88% <0%> (-2.46%)	9% <0%> (-2%)
... and 535 more

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[ajaaym]

merging this as codecov failure are unrelated to this change.