Combine uids for CpuTimePerUidFreq atom

The puller for the atom frequently fails. We believe it is due to the
size of the pulled atoms. No metric can be correctly build from this
atom.

To reduce the size, we apply transformations that we would likely
perform when analyzing this data. We combine times for all shared app
gids together and combine times for the same apps. We also exclude
isolated uids which are frequently recycled and removed from the
underlying data source.

If these optimization do not help, we will need to introduce different
atoms.

Bug: 157535126
Test: cmd stats pull-source 10010
Change-Id: I0d9a18c78f456e1b0f9d7ad322f15354ead9f02b