Set RTT capability properly in AccountEntry

When deciding whether the phone account should have the RTT capability
in TelecomAccountRegistry, properly set the mIsRttCapable cached
variable.
Also add a new permission to the telephony registry testapp.

Test: manual
Change-Id: Ic5e00102ed15e689949ae354b96ed34ad1cde288
Fixes: 123428485
2 files changed