fix: ignore XML value in DeviceConfig for RO flags

Make DeviceConfig.getProperty/getProperties return no XML values for
read-only aconfig flags. Only in-memory defaults are now returned.

In general, for read-only flags, this is not needed, since aconfig
codegen compiles away DeviceConfig reads. This addresses a problem with
exported flags, that read from DC even if read-only.

Flag: com.android.providers.settings.ignore_xml_for_read_only_flags
Bug: 345007098
Test: confirmed a read-only flag is not flipped in DeviceConfig
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:13ece1352d789289a2e049d8500f2a5ace0b949d)
Merged-In: I366c989f1aa8caabd00d0b502785b68142cba927
Change-Id: I366c989f1aa8caabd00d0b502785b68142cba927