Start enforcing javac warnings.

- Add new ways to ignore warnings for modules with warnings
- Enable -Xlint:deprecation and -Xlint:unchecked by default to
  prevent regressions.

Test: ./gradlew assembleDebug
Change-Id: I95f73afde042a2d61984f59ee3d3350a013c0044
diff --git a/paging/rxjava2/build.gradle b/paging/rxjava2/build.gradle
index f4c5308..2fd8caf 100644
--- a/paging/rxjava2/build.gradle
+++ b/paging/rxjava2/build.gradle
@@ -45,4 +45,5 @@
     inceptionYear = "2018"
     description = "Android Paging RXJava2"
     url = SupportLibraryExtension.ARCHITECTURE_URL
+    failOnUncheckedWarnings = false
 }