Disable APEX install on unsupported devices.

If the device ships with "flattened" APEXs, this will not be treated as
packages and therefore will not be updatable.

This CL also ensure that APEX files are only scheduled to be installed
as part of a staged session.

Fix: 124644545
Test: install succeeds on taimen (supported). Install fails on marlin
with following error.
$ adb install com.android.media.apex
Performing Streamed Install
adb: failed to install com.android.media.apex:
Exception occurred while executing:
java.lang.IllegalStateException: This device doesn't support the installation of APEX files
        at com.android.server.pm.PackageInstallerService.createSessionInternal(PackageInstallerService.java:497)
        at com.android.server.pm.PackageInstallerService.createSession(PackageInstallerService.java:450)
        at com.android.server.pm.PackageManagerShellCommand.doCreateSession(PackageManagerShellCommand.java:2587)
        at com.android.server.pm.PackageManagerShellCommand.runInstall(PackageManagerShellCommand.java:1011)
        at com.android.server.pm.PackageManagerShellCommand.onCommand(PackageManagerShellCommand.java:164)
        at android.os.ShellCommand.exec(ShellCommand.java:103)
        at com.android.server.pm.PackageManagerService.onShellCommand(PackageManagerService.java:20664)
        at android.os.Binder.shellCommand(Binder.java:887)
        at android.os.Binder.onTransact(Binder.java:771)
        at android.content.pm.IPackageManager$Stub.onTransact(IPackageManager.java:4689)
        at com.android.server.pm.PackageManagerService.onTransact(PackageManagerService.java:3698)
        at android.os.Binder.execTransactInternal(Binder.java:1026)
        at android.os.Binder.execTransact(Binder.java:999)

Change-Id: I6cc0c1d4cb143197cf4cc88a86dc6b758c6c70b2