Add trace slices to UnpackJob

Also decompose some methods into smaller methods, in order to get the
trace slice names matching the method names.

Bug: 429083945
Flag: com.android.documentsui.flags.use_material3
Flag: com.android.documentsui.flags.zip_ng_ro
Test: atest DocumentsUIGoogleTests:com.android.documentsui.services

Change-Id: I6b566b269103ebe3883a28131b1ffe17683cb7ab