applypatch: Add backup_source parameter to PatchPartition.

And set it to false when installing recovery image via applypatch. We
only need to back up the source partition when doing in-place update
(e.g. when updating a given partition under recovery). When installing
recovery image via applypatch, we won't touch the source partition (i.e.
/boot).

Removing the backup step also allows dropping the dac_override_allowed
permission. Previously it was needed due to the access to /cache.
Because applypatch runs as root:root, while /cache is owned by
system:cache with 0770.

Bug: 68319577
Test: Invoke the code that installs recovery image; check that recovery
      is installed successfully without denials.
Test: recovery_unit_test passes on taimen.
Change-Id: I549a770b511762189d6672a2835b6e403d695919