[Nix] Set PATH for android utilities

Summary:
Avoid user having to set this manually.

https://github.com/CommE2E/comm/blob/master/docs/dev_environment.md#enable-android-cli-commands

Test Plan:

nix develop
echo $PATH | grep emulator && echo "Yep, emulator is there"

Reviewers: abosh, varun

Reviewed By: abosh

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D4965