Make the SettingsProvider CTS test less flaky

The Settings_SystemTest.testSystemSettings() method implicitly relies on
certain specific keys being undefined in the Settings.System namespace
in order to run correctly.  However, failures parway through testing
would poison the settings db and leave those keys defined, causing
subsequent tests to potentially fail spuriously unless the device had
been wiped before re-running the test.

This CL adds a step to the test wherein it establishes the necessary
clean state explicitly before attempting the test.

Bug 7164713

Change-Id: Iafe3df4dd3322d0686936cb2adb519b7660b40a5