Fix Security Vulnerability in mediaserver

Crash on new CryptoPlugin::SubSample[].  numSubSamples
wasn't sanity checked, a malicious caller could pass
a very large number of subsamples causing the new
operator to fail which would crash the server process.
Add a sanity check to numSubSamples, which is defined
by the ISO-BMFF spec to be represented as unsigned 16
bits.

bug: 23718580
Change-Id: I36e18e60f2515289d1873640a3408c01e40a1174