Build DocumentsUI sources once

Put all the DocumentsUI sources and resources into an android_library
so that they are only built once and can always use a single package
name to refer to the resources once use_resource_processor: true is
enabled.

Bug: 294256649
Test: DocumentsUI DocumentsUIGoogle DocumentsUITests DocumentsUIUnitTests
Change-Id: I97387234c9e90fa2a68b9ee7e013dbb0f15c04ca