Fix the error of androidmk unittest

The unittest checking Android.mk path which is the relative path with
"ANDROID_BUILD_TOP". This can be fix with clean it.

Bug: 319563069
Test: 1. cd build/soong/androidmk/androidmk
      2. go test
Change-Id: Iaf6c331abeadfec3416387f416aaebc0ecfa8a21