Better API change management

Removes the current.txt file and prevents finalized API files
from being modified. Allows whitelisting of API check errors
to work around Doclava issues and API Council approved changes.

Also removes a lot of unused code from CheckApiTask.

Test: ./gradlew updateApi
Change-Id: Ic9e1800cbf8d0329e428c71be6d68e2f1e5abf4b
diff --git a/api/25.3.0.ignore b/api/25.3.0.ignore
new file mode 100644
index 0000000..1ef5739
--- /dev/null
+++ b/api/25.3.0.ignore
@@ -0,0 +1 @@
+3e22d56