[Custom handler] Remove custom handlers during uninstall

[Custom handler] Remove custom handlers during uninstall

[Issue#] N/A
[Problem] Custom handlers are not removed when widget is uninstalled.
[Cause] N/A
[Solution] Added new job to uninstall process: remove from appsvc and
from custom handlers db.
[ScmRequest] wrt-commons
Depends on: https://tizendev.org/gerrit/#/c/22499/
[Verification] Install custom_handlers.wgt from manual tests
from wrt-extra. Run widget, register protocol and content handlers.
Uninstall widget. Check if widget is removed from database (wrt_custom_handlers.db
and appsvc.db)

Change-Id: I2c84086f47c7f9ffcfa80905d16d10148dc90d48