Simpleperf: refactor dso.

Having DsoEntry and DsoFactory confuses me which part code should belong to.
This change merges the two into class Dso and makes things clear.
It is also a preparation for performance optimization in Dso.

Bug: 23387541
Change-Id: I41e773406a7f1582a11a18859df252ce8ea3acfa
10 files changed