Allow builds with costum toolchains.

Android bundles an OpenJDK-derived toolchain to avoid issues with
unsupported toolchains. For development / experiment purposes, this
CL the toolchain to be overridden via OVERRIDE_ANDROID_JAVA_HOME.

It is an error for OVERRIDE_ANDROID_JAVA_HOME to be set but not
point to a valid toolchain, but this error is not explicitly
checked for.

Bug: 38177295
Test: Treehugger

Change-Id: I72f641f560501e498f9c86a4380f19941fca11ad