Mock DisplayMetrics in NotificationManagerTest

DisplayMetrics are used in some downstream branches and need to be
mocked. This avoids merge conflicts in NetworkNotificationManagerTest.

Partial cherry-pick of change IDs:
I35d28c8df341dbbac2774026c6ca749e296c0482
I1aeed1c79e4a829d8829eb08224f9b21fafc50fe

Test: atest NetworkNotificationManagerTest
Bug: 176239013
Merged-In: I3c2563d4ae4e3715d0c6270344ba8f7ef067872f
Change-Id: I118ec1af615aef36293bf0b480389f766122ccab
1 file changed