Put app image objects in HPROF_HEAP_APP instead of HPROF_HEAP_IMAGE

Tools usually hide HPROF_HEAP_IMAGE and HPROF_HEAP_ZYGOTE classes,
so we probably don't want app classes in there.

Regression test is infeasible since we don't have a way to parse
hprof files from run-tests.

Test: test-art-host

Bug: 35762934
Change-Id: I56f4feb010a2e6cd290dfabdd8118e476098e40d