Refactor ActiveGestureLog and CompoundString in preparation for ProtoLog 2.0

We will need to send formatted strings to the ProtoLog api, so it is simpler to use CompoundString as the default string storage in ActiveGestureLog.

Refactored CompoundString to be able to be broken down into a formattable string.

Flag: N/A
Fixes: 313640579
Test: ran launcher, completed gestures, dumped gesture logs
Change-Id: I5f290d1860c6bac5047cbf361537829fbe986685