otautil: Move RangeSet implementation into rangeset.cpp.

Since it has grown much larger, users of the header shouldn't compile
and carry their full copies.

Also add missing header includes in imgdiff.cpp and imgdiff_test.cpp.

Test: mmma bootable/recovery
Test: recovery_unit_test; recovery_component_test; recovery_host_test
Change-Id: I88ca54171765e5606ab0d61580fbc1ada578fd7d