Stop using the `files` target for droidcore

This way if a BoardConfig.mk configures a specific image to exist (so it
doesn't end up as a folder on /system), but does not configure for it to
be created (like the device targets on AOSP that use a prebuilt), we
won't unnecessarily trigger the build system to build the contents.

Test: `m` before and after, comparing file lists
Test: check treehugger builds before/after
Change-Id: If0e4b958b3dfaa02771a5da70f970379635f904e