[DocsUI, Search]: Enable filtering by MIME type.

Adds filtering by MIME type to the loaders. Rearranges the order of
how filtering is done to make them identical in the folder loader and
search loader.
Expands the functionality of the TestDocumentsProvider to add fildering
by MIME types and the last modified time. Adds unit tests to cover the
newly added functionality.

Test: m DocumentsUIGoogle
Bug: 378590632
Flag: com.android.documentsui.flags.use_search_v2_rw
Change-Id: I4077853d85349814720bb3d4db9dcc9e0c96bc68