Improve several permission description Strings.

The MODIFY_NETWORK_SETTINGS and MODIFY_SOUND_SETTINGS permissions were
not descriptive enough. Improve the strings to enumerate exactly what
settings these permissions protect.

Fixes FOR-23.

Change-Id: Idd64738c21d879462a7e7e6c65ceefdc771f1bcd
(cherry picked from commit 29be6a63)