Remove uses of $USER

The definition of $USER is changing between inside the build and outside
-- on Linux, with the new sandbox I'm working on, it will always be
"nobody" inside the sandbox. (Mapping to the correct user outside the
sandbox with user namespaces, so file owners are still correct).

TMPDIR is always set by the build system to something that is unique to
the out directory -- it's currently $OUT_DIR/soong/.temp, and cleared
between builds. This means we can simplify some of the uses of $USER
within the art makefiles.

Bug: 122270019
Test: treehugger
Change-Id: I1277a7e9cc64759e787c0d4bf8e088f63d70be69
2 files changed