[Battery] Define layout rects as insets

In order to support RTL, it's a little bit simpler to define the layout
rectangles as insets from their edges rather than absolute edges. This
change adds support for reading RTL or LTR layout rects at the top level
battery layer.

Note that this is not RTL support, just getting this small refactor out
of the way to land RTL support next.

Test: manual using shell commands:
Test: adb shell cmd battery unplug
Test: adb shell cmd battery set level [1-100] # setting 0 will turn the phone off
Test: adb shell cmd battery set ac 1
Bug: 314812750
Flag: ACONFIG com.android.settingslib.flags.new_status_bar_icons DEVELOPMENT
Change-Id: Ic8b15286342a05e9c6ee2b371114e00c77500414