Restrict mixed build cquery to deps of depth 2.

This will be the list of targets directly being requested + additional
targets necessary for mixed_root. This reduces the size of the cquery
file by ~300Kb. Additionally, it removes unnecessary targets, making it
ever so slightly easier to find what one is looking for in cquery.out
for debugging purposes.

Test: build/bazel/ci/mixed_droid.sh
Change-Id: I6d0e337c9ab7c13388459dee16415be0ab3c0506