Make libsnapshot_flags_cc_lib available to recovery and ramdisk modules

Due to b/424439549, Soong wasn't enforcing that ramdisk and recovery
modules could only link against other ramdisk and recovery modules.
Add ramdisk_available, recovery_available and vendor_ramdisk_available
to libsnapshot_flags_cc_lib since it is used by ramdisk, recovery and
vendor_ramdisk modules to allow re-enabling the error in Soong.

Also add mode: "force-read-only" to prevent dependencies on the
modules that read flags at runtime since the flag storage won't be
available in ramdisk or recovery.

Bug: 424439549
Bug: 428274881
Flag: EXEMPT build only fix
Test: builds
Change-Id: I1858a1d6ad1d93adb9be867eeada56f7be4d3c59