Clear the setttings provider cache before test runs

The Settings class has a cache (Settings$NameValueCache)
which stores, in process, data read or written via the
SettingsProvider.

Currently we do not clear the cache between tests, which
means that tests isolation is lost because each test
shares a common cache with other tests. This CL forces
the cache to be cleared between tests.

Bug: 139449903
Test: atest SettingsProviderTest
Change-Id: Iebbb335fd56b1bd24cf0e2ab98a1f392e4b84609