Make OP_LEGACY_STORAGE stickiness configurable

OP_LEGACY_STORAGE is sticky for apps targeting <= Q.
This change makes this behaviour configurable via DeviceConfig, by
introducing a new property "legacy_storage_op_sticky" to existing
namespace "storage_native_boot". If the property is set to true, then
we get the default behaviour (app-op sticky for SDK<=Q). If it's set to
true, then the app-op is not sticky for SDK<=Q.

Apps targeting > Q remain unaffected: always not sticky.

Test: manual:
    * adb shell dumpsys appops --package com.android.vending
    * Observe LEGACY_STORAGE mode=allowed
    * adb shell device_config put storage_native_boot legacy_storage_op_sticky false
    * adb reboot
    * adb shell dumpsys appops --package com.android.vending
    * Observe LEGACY_STORAGE mode=ignored
Bug: 151735608
Change-Id: I06d115a0c85c44b5a6d1054f74a00d8fa674dfa7