Add script to "shrink" SystemUiRavenTests

SystemUiRavenTests has a lot of kotlin source files, so it's slow to build.
Which is painful when you want to run it after updating ravenwood code
that SystemUiRavenTests depends on. (example: junit-src/)

This script basically removes the test files SystemUI/multivalentTests
that don't have @EnabledOnRavenwood. But if we actaully remove the file,
soong would re-generate the ninja file, which will take a long time,
so instead this script will truncate the files.

This script will also tell git to ignore these files, so they won't shw up
in `git status`.

Flag: EXEMPT host side test change only
Bug: 292141694

Test: Run `atest SystemUiRavenTests` after shrinking it,
      then undo it.
Change-Id: I61447be9353b8bb6d0623853785d418266681125