Fix java.lang.NoSuchFieldError on R$styleable

Some unused styleable was removed by proguard that TestCase can't refer
at runtime.
Refactor and move styleable from docsui to TestCase res if necessary
(Prevent unused resources removed by proguard)

Note: GridItem default color is transparent(res/color/item_doc_grid_tint.xml)
Remove useless GridItem testCase due to the color is not relatie to theme

Fixes: 131835503
Fixes: 131735635

Test: atest DocumentsUIGoogleTests

Change-Id: I20b0d823c0d166466f18023928021ced58e32c21