WM Tests: Use a separate WindowManager instance per test

Fixes a bunch of flakes, where the WindowManagerService instance
was reused between tests, which caused delayed callbacks from a
previous test affecting state of a future test.

Also introduces a DexmakerShareClassLoaderRule to manage
the 'dexmaker.share_classloader' property instead of sprinkling
error prone System.setProperty() invocations all over the tests.

Change-Id: Ic9445d1b2cef594e79365c425632aabced6343a9
Fixes: 76111404
Fixes: 75991352
Fixes: 75991878
Fixes: 75992153
Test: atest services/tests/servicestests DexmakerShareClassLoaderRuleTest packages/SystemUI/tests packages/SystemUI/shared/tests