Allow explicitly setting the camera component launched by double-press power.

Using an implicit intent at the moment of picture-taking
usually goes unnoticed. But immediately after installing a
new camera, this behavior becomes incredibly frustrating to
users as they are presented with a puzzling resolver dialog
(or in the case of the secure camera, the authenticator).
And if, at this moment, the user chooses to make one of the
options a default, it's almost impossible to figure out how
to change this setting.

As a result, many OEMs simply hardcode the camera gesture to
launch a specific preinstalled camera, but this is poorly
supported by AOSP, leading to duplicate implementations and
bugs. This patch routes all camera intents in System UI
through a single utility class, creating a convenient spot
to insert a resource that contains the OEM's default
preinstalled camera app.

Note that this does not affect implicit intent resolution in
any way; any app may create a chooser for, e.g.,
MediaStore.INTENT_ACTION_STILL_IMAGE_CAMERA and allow the
user to pick from the available cameras.

Bugreport/dumpsys output to look for:
$ adb shell dumpsys activity service com.android.systemui | grep -C3 'Camera gesture' | tail -3
    Camera gesture intents:
       Insecure camera: Intent { act=android.media.action.STILL_IMAGE_CAMERA }
       Secure camera: Intent { act=android.media.action.STILL_IMAGE_CAMERA_SECURE flg=0x800000 }
       Override package: null

Bug: 171807357
Fixes: 154218868
Test: atest SystemUITests
Change-Id: I2c0033e52c8a3963768d29f2e76e555d405aaa7e