Test if allocation supported; @removed clean up.

SM.allocateBytes() doesn't offer a clear way to detect if a failed
request could ever succeed.  (For example, we can never work with
pipes, or files on an unsupported storage device.)  So give
developers a first-class API to test if allocation is supported.

If the underlying filesystem doesn't support fallocate(), fall back
to ftruncate() instead of failing completely.

Clean up @removed APIs that were refactoring during API 26 review
process.

Remove support for storing downloads on the /cache partition, which
doesn't exist on many devices.

Bug: 63057877
Test: bit DownloadProviderTests:*
Test: bit DocumentsUITests:com.android.documentsui.services.CopyJobTest
Test: bit DocumentsUITests:com.android.documentsui.services.MoveJobTest
Change-Id: I85d42a1a7240034b4f2a6f359011ac182bdce36e