Only mount dist dir to soong sandbox if it exists

The dist dir is created late in the build process. If a
soong sandbox attempts to mount it before it's created
the sandbox will create an empty file in its place.

Test: lunch aosp_cf_x86_phone-userdebug && make -j dist
Change-Id: Ie6513bf702de6e6322c78753d406d70ea3ccc04d