Fix nested minijail0 execution

We've got an internal testcase that uses minijail0 inside an Android.mk
rule. That was failing since we turned on the linux sandbox, as /proc
was mounted read-only, which prevented setting up the uid/gid mappings
for a child namespace.

Fixes: 122985455
Test: treehugger & forrest of breaking build
Change-Id: Ia77a91a7f4eeeb8a24e84075d8272287f5087587