Split the original U64/UBE64 into two groups: U63/UBE63 which could be
represented by long primitive directly and U64/UBE64 which should be
represented by BigInteger class.
Also fix the endianness-related issue to support both of big-endian or
little-endian input ByteBuffer, and writeToBytes API outputs the bytes
array in appropriate order then.
Bug: 163492391
Test: atest android.net.util.StructTest --rerun-until-failure
Change-Id: Ie9c07fac6dcfceb8efdf1d6b56ce6ff1e845f477
f4cb6d6789