Move boot image oatdump phony rules into Soong

The boot image oatdump rules depend on the details of dexpreopting
the boot image.  Instead of exporting all of the necessary values
to make, move the rules into Soong instead.  Also removes the
ART_DUMP_OAT_PATH variable, and moves the output to
out/soong/boot.*.oatdump.txt.

Test: m dump-oat-boot
Change-Id: I055b1c39918ba3425c8393b3e1b5359df055472a