Build Flag declarations for setting SVN (Software Version Number) per device

Currently we set SVN per device in device specific device-<name>.mk
files. Make it easier to set SVN values per device through build flags
instead.

Ignore-AOSP-First: New flags that will be pushed to aosp as part of next
release

Bug: 332726030
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e8ebe36e3293bfdf33fdd6770e4c5290b3d6aa69)
Merged-In: Idc32b3af68074820c8da13c045c205ea32e894aa
Change-Id: Idc32b3af68074820c8da13c045c205ea32e894aa