Add a GetMappedPackageLength to Updater

Corresponds to GetMappedPackageAddress.

There is at least one custom device recovery with an extension to copy
the package somewhere else as a backup. Ability to do this was removed
in change ag/955273.

Test: lunch cf_x86_tv-eng; make
Test: atest recovery_unit_test recovery_component_test
Change-Id: I0e61d8a8839c47721874526504ea03b9ca7bafa9