[frameworks/native] Make MemoryReporter::TraceValue own its string

TraceValue had an unowned pointer to a const char (string). But there
was no guarantee that the string lived long enough for TraceValue's use.
Switch to an owned string type.

Bug: 305919946
Test: adb shell dumpsys SurfaceFlinger
Change-Id: I30b990771cc7f3ccf2769efc5aafb68c957eca9f