Add hardcoded Wear settings to Settings.java and SETTINGS_TO_BACKUP

Some settings used in Wear OS aren't in Settings.java, instead, they are
just hardcoded strings in various places. To add them to
SETTINGS_TO_BACKUP, they need to be in Settings.java. So, this CL adds
them to both Settings.java and SETTINGS_TO_BACKUP

Unfortunately, most of the usages of these strings are outside the
frameworks/base module. So, I will have to 'fix' them to use these new
constants in Settings.java instead of hardcoded strings in follow-up
CLs.

Bug: 228159084
Defer-CP-To-TM: 228159084
Test: Flashed a prototype build with these changes to a test watch
Change-Id: I6bebb0c2b93b34f80d56395caacc00f20a506ca0
(cherry picked from commit 24c2d4fb10e04e9b69ea4e3f4e82ec11c3eb4d08)
(cherry picked from commit 162b295dcdc797577af00ee5640e373abb907954)