Allow mainline IPsec(IKE) to statically include @PolicyDirection

@PolicyDirection was hidden API defined in IpSecManager and
mainline module IPsec(IKE) needs to depend on it.

To remove this hidden API dependency, this commit:
- Moves definition of @PolicyDirection to a separate class
- Creates sharing filegroup framework-ike-shared-srcs for mainline
  IKE and include PolicyDirection.java

Bug: 146360859
Test: build, flash, boot
Test: atest FrameworksIkeTests
Change-Id: Ic6d7c06d4b92e16a9a65430365c9acc73932147b