Create loader for Power HAL services

Move logic to load underlying Power HAL services to a static class in
services/powermanager, with tests. This can now be reused by
BatteryStatsService and PowerManagerService, removing the dependency
between them.

Bug: 150878220
Test: atest powermanager_test
Change-Id: I52eec130811d49111526f3e152faf4302518c5a8
diff --git a/services/powermanager/tests/Android.bp b/services/powermanager/tests/Android.bp
index 65cde030..1cf170e 100644
--- a/services/powermanager/tests/Android.bp
+++ b/services/powermanager/tests/Android.bp
@@ -12,10 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+
 cc_test {
     name: "powermanager_test",
     test_suites: ["device-tests"],
     srcs: [
+        "PowerHalLoaderTest.cpp",
         "PowerHalWrapperAidlTest.cpp",
         "PowerHalWrapperHidlV1_0Test.cpp",
         "PowerHalWrapperHidlV1_1Test.cpp",