Dispatch keys to a device specific key handler

Injects a device key handler into the input path to handle additional
keys (as found on some docks with a hardware keyboard).

Configurable via overlay settings config_deviceKeyHandlerLib and
config_deviceKeyHandlerClass.

Change-Id: I6678c89c7530fdb1d4d516ba4f1d2c9e30ce79a4

Signed-off-by:  Paul Beeler <sparksco@gmail.com>

Fix build from mis-merge
Revert "Allow purging of asset bitmaps"

This reverts commit 8a6d11ac68fc1a5224e62846ce8e9d01c138397e.