Change CellSignalStrengthGsm.mTimingAdvance to greylist-max-p

Hidden APIs whose access has been restricted in Q should be assigned
maxTargetSdk=P. Fix a field in CellSignalStrengthGsm which was assigned
maxTargetSdk=O.

Bug: 124354830
Test: check out/soong/hiddenapi/hiddenapi-flags.csv
Merged-In: I025887bf7fbc513da155bfe12919cfea9bd9c5fa
Change-Id: I025887bf7fbc513da155bfe12919cfea9bd9c5fa
(cherry picked from commit 2959b5c1)