Follow-up to Ie62a4486

Clear the flag only when all digits are removed.

TESTED:
- Manually type "123#"
-> DOES trigger special-char-sequence handling

- Have Intent with "tel:555", remove all digits with back press, type 123#
-> DOES trigger the handling

- Have Intent with "tel:555", remove all digits with back long-press, type 123#
-> DOES trigger the handling

- Have Intent with "tel:555", dial this, hang up, and type 123#
-> DOES trigger the handling

- Have Intent with "tel:123%23"
-> Does NOT trigger the handling

- Have Intent with "tel:123", type #
-> Does NOT trigger the handling

- Have Intent with "tel:123%23%23", remove the last #
-> Does NOT trigger the handling

- Have Intent with "tel:123%23", remove the last # and type #
-> Does NOT trigger the handling

- Have Intent with "tel:123%23", change locale, relaunch the dialer
-> Does NOT trigger the handling

Also did the same with hardware keyboard.

Bug: 6726923
Change-Id: Idea7ae599e3a217ad656a304fbae26746d9f3284