Made CheckJNI check varargs when possible

Several JNI functions such as NewObject and Call*Method take a variable
number of arguments. This patch will make CheckJNI do (limited) dynamic
validation of these arguments. Currently it is limited to checking that
objects are valid and that no value types have illegal values.

Bug: 20344596
Change-Id: I1f81d2bdd80215e0007fc636bac27d5bcc2dba96