2/ Do not access SettingProvider in test code

- Refactor OneHandedSettingsUtil.java
- Remove redundant test for SettingProvider

New policy: b/175024829
Settings key: <one_handed_*> is not readable.
From S+, new public settings keys need to be annotated
with @Readable unless they are annotated with @hide.

Recent changes :
ag/13764747 Remove @Readable for hidden fields added in S

Test: atest WMShellUnitTests
Bug: 183064974
Change-Id: I85bf3680cbcdba4c85aa96de058bc85cfabead46