frameworks/base/wifi: Add MOVED.txt

Document where the files have been moved to.

Bug: 137323948
Test: compiles
Change-Id: Ib1a017a71c265eba00eae44663fd225ce1bec0f8
Merged-In: Ib1a017a71c265eba00eae44663fd225ce1bec0f8

wifi/MOVED.txt

+Source code and tests for Wifi module APIs have moved to
+packages/modules/Wifi/framework.
+
+- frameworks/base/wifi/java -> packages/modules/Wifi/framework/java
+- frameworks/base/wifi/tests -> packages/modules/Wifi/framework/tests
+
+What remains in frameworks/base/wifi are Wifi APIs that
+are not part of the Wifi module.