android.provider.Setting test utilities

Add a way to fake values for settings in tests. Since the content
provider is cached in the NameValueCache, there is one static
FakeSettingsProvider that passes through all values to the
real SettingsProvider by default. Values that are required for
the test can be acquired and locked for the duration of the test
easily.

Test: runtest systemui
Change-Id: Ibc31ac8509fb31a22c522358a9c1bae6ec63553b