SECURITY: Make token comparison constant time

It seems like our token comparison would be vulnerable to timing
attacks. This will take constant time.

Bug: 61346
Change-Id: I2a9e89120f7092015495e638c6fa9f67adc9b84f
(cherry picked from commit 74ce391e9c115320e74c06772ee4fcb610ae33c4)
Merge "SECURITY: Make token comparison constant time" into REL1_21