Show every file in the picker, and start it at the last document

[andiwand]

Users report that they cannot find their documents in the file picker.

What changes

1. The picker shows all files again. Since Redesign: Material 3, recent documents as the landing screen, and actions at the thumb #547, findDocument gave the picker an EXTRA_MIME_TYPES filter. The picker does not grey out a file that fails the filter. It leaves the file out, so the user thinks that the file is not on the phone. A provider can report a type that the core's table does not spell, and then a document that the app can open disappears. The filter is gone, and the intent uses only */*, as it did before Redesign: Material 3, recent documents as the landing screen, and actions at the thumb #547.
2. The picker starts in the folder of the last document. findDocument gives the newest recent document as EXTRA_INITIAL_URI. Only a document uri works, so the code skips an entry that came from ACTION_VIEW.
3. A hint under the Open button tells the user about the picker's drawer (☰) and about opening the file from a file manager. Only this new string is translated. The 15 strings that main is already missing in each language are out of scope.

Tests

On an API 36 emulator:

• With no recent documents, the picker opens in Downloads and lists every file.
• After the app opened Documents/Work/report.odt, the next picker opened in Documents › Work.
• spotlessCheck, assembleDebug, lintProDebug, testProDebugUnitTest and LandingTests (16 tests) pass.

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