[DOC] document behavior and workarounds for executables in APKs

**URL**
None yet.

**Problem**
Executables in APKs are supported but there are a number of gotchas (the biggest being that if your binary is not named `lib*.so` it won't be installed by packagemanager). We should document the things people need to know to use these, and also recommend simpler alternatives.

See https://issuetracker.google.com/152645643.

The issue names no documentation file or test. Start with the linked Google issue and the existing Android NDK documentation; document executable APK behavior, the lib*.so naming and package-manager limitation, known workarounds, and simpler alternatives.

Written by the indexing model from the issue text.