Add new 'explicit GC' policy to StrictMode.

This change adds the policy but offers no public way to enable it. A
follow-up change will expose the detect/permit methods in the API and
change detectAll to enable it.

This new policy can only be triggered through the libcore BlockGuard API.

Bug: 3400644
Test: cts-tradefed run cts-dev -m CtsLibcoreTestCases
Test: cts-tradefed run cts-dev -m CtsOsTestCases
Change-Id: I2e7f34ce010c78d6a5a7ac85512c045bfb13d204
Merged-In: Ieebe4db747902246968d6382bbc9cee0e539af85