Allow access to the "refresh_rate_switching" system property

Bug: 147233255
Bug: 136592946
Bug: 138261472

Test: Ran with the patch applied, confirmed surface flinger can access
the system property.

Change-Id: I259a488399c5e698de384322852ea81ea1a96e7d
Merged-In: I259a488399c5e698de384322852ea81ea1a96e7d
Exempt-From-Owner-Approval: Approved internally
2 files changed