Deal with unknown signal strength level comes from Telephony

According to TS27.007 8.5, GSM level 99 represents
not known or not detectable. Bluetooth doesn't take it into
consideration, so HF unit cannot display correct signal level.

Add judgement for this special case so that HF unit can match
with status bar of phone.

Bug: 28992746
Test: make
Change-Id: I951c808472dff11af5584ac66c06386d8c601bbb
Merged-In: I951c808472dff11af5584ac66c06386d8c601bbb
(cherry picked from commit a7c269cb)