[Filesystem] Fixed Blob to Uint8Array conversion

[Filesystem] Fixed Blob to Uint8Array conversion

Fix inspired by this answer: https://stackoverflow.com/a/63920556/11502478

[Verification] in Chrome console:
// assume having wav file on the device
input = tizen.filesystem.openFile("documents/sample1.wav", "r")
blob = input.readBlob()  /// Blob {size: 1073218, type: ""}

output = tizen.filesystem.openFile("documents/sample1_output.wav", "w")
output.writeBlob(blob)
output.close()

output_test = tizen.filesystem.openFile("documents/sample1_output.wav", "r")
blob_test = output_test.readBlob()  /// Blob {size: 1073218, type: ""}

console.log("Verification passed: " + (blob.size === blob_test.size))

input.close()
output_test.close()

TCT (filesystem, deprecated) passrate 100%

Change-Id: I5e4beea31d69430e9dbe44a7899cf675e779c7f8