Make Display.Mode.isSynthetic TestApi

To use in CTS (FrameRateOverrideTest) for vrr device with synthetic
modes.

Bug: 353142444
Flag: TEST_ONLY
Test: atest FrameRateOverrideTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:62fb8f943b0251ecec710f14b63e562ac63ce0dd)
Merged-In: Ic28956ac65f9642b9a78a31fff0111e28c6a69e0
Change-Id: Ic28956ac65f9642b9a78a31fff0111e28c6a69e0