Minor clean up to install.cpp.

- Move some macros / constants into functions;
- Remove unneeded #include "minui/minui.h";
- Remove two dead constants (DEFAULT_{FILES,IMAGES}_PROGRESS_FRACTION).

Test: mmma bootable/recovery
Change-Id: Ib808f14b7569e06e23a8a7cc9b2d4e9aa5469de1