Add support for streaming hprof dumps.

Add support for streaming hprof dumps.

This adds the dumpHprofDataDdms method, which generates the hprof dump
in RAM and then spits the whole thing at DDMS.  The idea is to avoid
touching /sdcard, since not all apps have permission to do that.

This rearranges hprofShutdown() a fair bit.  It used to re-use a context
struct, saving interesting bits to local variables before zapping it;
now we just create a second context struct and free both at the end.

For bug 2092855.