Fix test action that tampered file at wrong offset

The implementation in writeZerosAtFileOffsetOnMicrodroid specifies the
offset of the output file by "skip" of "dd". But actually, "skip" is for
"if=", while "seek" is for "of=". The test expectation did not fail
coincidentally.

Bug: N/A
Test: testWriteFailedIfDetectsTampering
Change-Id: I89c9c96d250f4374e51a01c986aa1289b24ac236