Force apps to request permissions for teamfooding.

To support teamfooding of the new storage privacy features coming
in Q, we need apps to request new AUDIO/VIDEO/IMAGES permissions, but
most of those apps are prebuilts that won't land updates until
several months in the future.

So add system properties so teamfooders can "force" apps to request
these permissions, making them work on Q builds.  Only takes effect
when isolated feature is enabled, and guarded with STOPSHIP to ensure
we remove it.

Here's a typical set of commands to use with this CL:

    adb shell setprop persist.fw.force_legacy 1
    - or -
    adb shell setprop persist.fw.force_audio com.google.android.music
    adb shell setprop persist.fw.force_video com.google.android.apps.photos
    adb shell setprop persist.fw.force_images com.google.android.apps.photos,com.google.android.apps.messaging
    - then -
    adb shell setprop persist.sys.isolated_storage 1
    adb reboot

Bug: 118504670
Test: manual
Change-Id: I631819648334994255256b6046bb4c8aec07ce3a