bp2build: support genrule $(location) and $(locations)

Soong genrules support $(location) and $(locations) cmd variable
shortcuts without labels. The shortcut is for the location of the first
tool module from the concatenation of the tools and tool_files
properties.

Bazel doesn't support this shortcut, so the bp2build mapping needs to
support it.

Documentation:
https://cs.android.com/android/platform/superproject/+/master:build/soong/genrule/genrule.go;l=95-96;drc=316e07c593ab66599c74725cf482dedbb32b2875
Code: https://cs.android.com/android/platform/superproject/+/master:build/soong/genrule/genrule.go;l=236-246;drc=316e07c593ab66599c74725cf482dedbb32b2875

Test: build_conversion_test.go
Test: TH

Change-Id: I8aa98ae460af3a3fbb0a7835572518680dc7ade1