Fix PendingIntent hijacking for adb notifications.

Use an explicit intent and set PendingIntent.FLAG_IMMUTABLE to prevent
someone from modifying the intent from PendingIntent.send(...).

Bug: 153356209

Test: atest AdbNotificationsTest
Test: In bug, install and launch the PoC apk and give it notification
permissions. Then, with USB/Wifi debugging enabled, disconnect and connect
the device to create the adb notification. the PoC apk should not have
permission to display information from
content://com.android.settings.files/my_cache/NOTICE.html.

Change-Id: Ie49aa3cf9b33168cf1435fc2427e95aac7f4609b