Don't show any trust granted message if passed an empty string

If the trustGranted call has flag DISPLAY_MESSAGE and
message is intentionally set to an empty string "", then
don't show ANY trust granted message. If the message is null
or DISPLAY_MESSAGE is not set when trust is granted,
then the device will show its default trust granted string.

Test: atest KeyguardIndicationControllerTest
Test: manually grantTrust with the FLAG_GRANT_TRUST_DISPLAY_MESSAGE
and an empty string. See that device has trust granted and doesn't
show any keyguard indication message.
Fixes: 243075236
Change-Id: Ic31a1758e9baa27b236f09ba8e656d15dd04e7f0