Read clipboard item mimetype from description

Currently we construct the clipboard model by calling
contentResolver.getType on the main thread, which can cause problems
because that call can hang.
This change switches to reading the mimeType set in the clip description
instead (which should be set correctly per-item).

Bug: 357197236
Flag: com.android.systemui.clipboard_use_description_mimetype
Test: manual with flag on/off, atest ClipboardModelTest
Change-Id: I78550105f9c4f9b3ee1cf69a732b075397cdf4ac