chore(accessibility): adjust TestableWindowManager#getCurrentWindowMetrics

In #getCurrentWindowMetrics, it should return copied
windowBounds/windowInsets instead of the exact cached objects, in case
that if there is any calculation or changes on the returned bounds/insets
the value in windowManger would be changed and thus affect the tests.

Bug: 431922747
Flag: EXEMPT test fix
Test: atest systemui/accessibility/*
Change-Id: Ib53928e86b1b86ddf8fba3bfa18dee02542209eb