Limit the accepted length of SIM PIN to 8 digits

Adding the upper limit results in the correct error message being
displayed to the end user - "Type a PIN that is 4 to 8 numbers."

Limiting the SIM PIN length to between 4-8 digits is part of the
GSM SIM security requirements.  See GSM 02.17 version 5.0.1,
Section 5.6 PIN Management.

Bug: 31331481
Test: 1. Insert a SIM card with SIM PIN enabled
      2. Start the device
      3. Enter a SIM PIN with a length > 8
Change-Id: Ie5fba1c79074910b0bfd54df51e4412bbf32f5fe