Use `SystemProperties.get` for app compat check.

The check for `bionic.linker.16kb.app_compat.enabled` is changed from using `SystemProperties.getBoolean` to `SystemProperties.get` and comparing the string value to "true".

Test: Set the compat mode and see that warning doesn't appear.

m FileSystemUtilsTests
adb install -r out/host/linux-x86/testcases/FileSystemUtilsTests/app_with_4kb_elf_no_override.apk
adb shell setprop bionic.linker.16kb.app_compat.enabled true

Bug: 438503313
Flag: EXEMPT bug_fix
Change-Id: I02413af5c85c265c8cd6355acfef6440d82f602b