Permit dropping caches from the shell through sys.drop_caches.

*   Permits setting the sys.drop_caches property from shell.
*   Permits init to read and write to the drop_caches file.
*   Can only be set to 3 (drop_caches) and 0 (unset).

Bug: 178647679
Test: flashed user build and set property; no avc denials.
Test: flashed userdebug build and dropped caches w/o root.
Change-Id: Idcedf83f14f6299fab383f042829d8d548fb4f5d
4 files changed