applypatch: Add testcases for applypatch executable.

Refactor applypatch/main.cpp into libapplypatch_modes so that we can add
testcases.

Some changes to applypatch/main.cpp:
 - Replace char** argv with const char**;
 - Use android::base::Split() to split ":";
 - Use android::base::ParseUInt().

Bug: 32383590
Test: Unit tests pass, install-recovery.sh works.

Change-Id: I44e7bfa5ab717d439ea1d0ee9ddb7b2c40bb95a4