modules: lvgl: simplify sys_heap memory size configs

Changes the Kconfig symbols for the sys_heap based memory management option
and removes the notion of `BLOCKS` with a more concise single
`LV_Z_MEM_POOL_SIZE` option. Also adds `LV_MEM_CUSTOM` to the lv_conf.h,
since in any option the memory management is considered to be custom.

Signed-off-by: Fabian Blatz <fabianblatz@gmail.com>