Increase device name limit in the LE advertisement

The advertise data refactor:
https://android-review.git.corp.google.com/#/q/topic:btif-ad-data-refactor
introduced a change in behaviour. Previously, the device name was
included in full, or truncated if the device name was longer than the
advertisement. After the patch, name longer than 18 chars are truncated.

Increase the allowed device name length to 26 - the maximum size it can
get for legacy connectable advertisement.

Bug: 37870615
Test: none
Change-Id: I3464ffae32e73cefa10142c2f0509d9139475ddd