Fix incorrect interpolation of active bucket for partial requests.

Just because the bucket is active (ie. overlaps with 'now') doesn't
mean the request necessarily covers the entire bucket.

For example a bucketDuration=5 with a bucket from curStart=1 to
curEnd=6 with now=4 and requests:
- from start=0 to end=3 --> overlap=[1,3] --> bucketSpan=2
- from start=2 to end=3 --> overlap=[2,3] --> bucketSpan=1
- from start=2 to end=5 --> overlap=[2,4] --> bucketSpan=2
- from start=2 to end=7 --> overlap=[2,4] --> bucketSpan=2
and the previously correctly handled cases:
- from start=0 to end=5 --> overlap=[1,4] --> bucketSpan=3
- from start=0 to end=7 --> overlap=[1,4] --> bucketSpan=3

We fix this by bounding the end of buckets to now, this means
that bucketDuration is no longer a constant, but it simplifies
all the rest of the logic.

Test: builds, atest
Bug: 143338233
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I642b57f7b8486071ad7808fd9b901859923b6d25