Add Strings page to AHAT

The strings page shows all string instances and their values, similar to allocations -> java.lang.String.
Additionally, it can group strings by value, and sort by count of dupes and by count * size of each string instance's bytes.

Example: shortn/_JFa5k5XNoW
(showing http://localhost:7100/strings for a random YouTube heap dump)

Flag: EXEMPT HOST_MODE
Change-Id: I352dc16f2979bedfee4e5889ea5d91a45308d83b