Export soong_ui if available

Check if the Soong bash wrapper is available, e.g.,
we are building in the unified Android-Trusty tree,
and export it to the build system as the SOONG_UI
variable.

Bug: 285203365
Test: build.by qemu-generic-arm64-test-debug
Change-Id: I6c47cf530cc8002e34507a57bb964edf3d16dfc7
1 file changed