[Nix] Default value of ANDROID_HOME

Summary:
Prefer to use ANDROID_HOME as it's the new canonical way to describe
the installation path of the Android SDK. Also alias the value to
ANDROID_SDK_ROOT for compatibility with certain tools such as sdkmanager.

https://linear.app/comm/issue/ENG-2274

Test Plan:

nix develop

echo $ANDROID_HOME
echo $ANDROID_SDK_ROOT # should match, and point to sdk installation

Reviewers: atul, ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5656