Bug 988768 - [Sora][Message][MMS] Can't send messages with an attachment...

... between 295 and 299KB r=schung

This changes the default setting in Gaia from 300KB to 295KB. This also
substracts 5KB from the setting taken in Gecko.

(cherry picked from commit f250750c9b8381560fa33b2383737736e4dbdff5)

Conflicts:
	apps/sms/js/settings.js
	apps/sms/test/unit/mock_settings.js
	apps/sms/test/unit/settings_test.js