Add Light/Dark theme for bugreport warning dialog

Previously added "android:theme="@android:style/Theme.
DeviceDefault.DayNight" to the application tag in this manifest file to
enable dark theme for bugreport info.
BugreportWarningActivity is nested within this application tag, so
assumed all the child tags to have DayNight theme.

android:theme="@android:style/Theme.DeviceDefault.Dialog.Alert" shows
the dialog always as Dark mode.

android:theme="@android:style/Theme.DeviceDefault.Light.Dialog.Alert"
shows the dialog always as Light mode.

This bug is not easily notice-able as "Do not show this again" checkbox
is checked by default for this dialog. Unless one wipes data partition,
this dialog doesn't come up even after flashing new builds.

Bug: 141663298
Test: Normal (light mode) Trigger interactive/full/wifi bugreport.
      Click on bugreport finished "Share" notification.
      Light mode warning dialog should appear. (with white background)
      Uncheck the "Do not show this again" checkbox to test with dark
      mode.
Test: Go to Settings. Turn on Dark theme.
      Trigger interactive/full/wifi bugreport.
      Click on bugreport finished "Share" notification.
      Dark mode warning dialog should appear. (with dark grey background)

Change-Id: I200c592608261665e4c2416303f573522c6bc59a