Increase challenge size in tests.

VTS tests were currently passing a challenge size of 32 in all cases.
However, the server currently sends a challenge of length 40, which may
or may not change in the future. A 64 byte upper limit provides a
standard size along with flexibility in case the challenge format
changes in the future.

Test: atest VtsHalRemotelyProvisionedComponentTargetTest
Change-Id: I678bb915f139e4c23354180870a66ce33a9cfd8c