Declare DecorInsetTestsBase$TestActivity in AndroidManifest

The declaration was removed in a previous CL because the implementations
of DecorInsetTestsBase were all moved out of
CtsWindowManagerDeviceTestCases.

However, InsetsParamsTest still uses the activity. This CL brings the
declaration back.

Fix: 380418196
Flag: com.android.window.flags.enforce_edge_to_edge
Test: atest InsetsParamsTest
Change-Id: I19a6f1a469ed92d6e19bbb363c47d894c69e5dc9