Increase poll max size, avoid performance issue when batching

Vts polls sensor events with max size of 1, which can cause
performance issue when batching. Increase it to 64 to help eliminate
potential performance issue.

Note this CL has no change in behavior of test.

Test: VTS target test pass on several devices
Change-Id: I04a6791f407ec0a54eb6cd8df4d4ef41e8a89c12