Fix out-of-bounds write past end of buckets on long transactions

For some values of max_time_bucket, (max_time_bucket-1) /
time_per_bucket will end up being num_buckets, which is one element past
the end of the buckets array.

Test: adb shell binderThroughputTest -- -m 500

Change-Id: Ie3933df6e563854b375604368c06dbd33697250a