CtsTransitionTestCases: add parent style to the app theme

The issue b/139458376#comment9 indicates the missing value
(android:attr/windowTitleSize) in theme causes the error on activity
initialization. And this might be a cause of the flakiness.

This CL adds a parent style to the app style theme so that the app style
theme contains the missing value correctly.

Bug: 139458376
Test: Ran CtsTransitionTestCases locally and passed
Test: Forest passes (Run ID: L67000000358627313)
Change-Id: Ie4f0fa00013c2f23c3526a2aa0585687f69773eb
1 file changed