Don't assume incremental fs exists on R launch devices

With R launch devices, OEMs were allowed to either ship incfs as a
module or built in to the kernel. If they chose to build it in, then
when we test GSI/GKI, incfs is not present so incfs tests all fail.

Since we cannot test for GSI and R launch combo, do not assume incfs on
R launch devices

Bug: 287545585
Test: atest CtsIncrementalInstallHostTestCases passes on a S or above
device
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:01784fffd99341f4e0d527b2a9ad984980745970)
Merged-In: I1c6c7bf15eee5a6f1a1eb6fd3381816d73550413
Change-Id: I1c6c7bf15eee5a6f1a1eb6fd3381816d73550413