drop APPLY_CACHE and refactor APPLY_EXT

Drop support for sideloading OTA packages of the cache partition (a
half-solution that's long since been deprecated by "adb sideload").
Refactor the code to sideload OTA packages from SD cards: remove the
installation code from the file browser.

Change-Id: Id0dff6b27c4a5837546f174f50e2e1d0379c43db