Make validateReadData smaller by factoring out some code

Increasing the size of validateReadData made read* benchmarks perform
poorly.
It looks like this splitting the added code out to own method
recovers that lost performance.

Flag: EXEMPT bug fix
Test: atest CorePerfTests:android.os.ParcelPerfTest#timeReadLong
Bug: 434246899
Change-Id: I79b2d39b5b8ac248b43760cb5508b890cfacd094